Of the five senses, vision gives us the most information about the world around us. But we can see the world around us only because light enters our eyes. So, we begin the study of light, or optical (Greek optikos - visual), phenomena, that is, phenomena associated with light.
Watching Light Phenomena
We encounter light phenomena every day, because they are part of natural environment in which we live.
Some optical phenomena seem to us a real miracle, for example, mirages in the desert, auroras. But you must admit that more familiar light phenomena: the sparkle of a dew drop in a sunbeam, a moonlit path on the water, a seven-color rainbow bridge after a summer rain, lightning in thunderclouds, twinkling stars in the night sky are also amazing, because they make the world around us beautiful. full of magical beauty and harmony.
Understanding Light Sources
Light sources are physical bodies whose particles (atoms, molecules, ions) emit light.
Look around, refer to your experience - and you will no doubt name many sources of light: a star, a flash of lightning, a candle flame, a lamp, a computer monitor, etc. (see, for example, Fig. 9.1). Organisms can also emit light: fireflies are bright points of light that can be seen on warm summer nights in forest grass, some marine animals, radiolarians, etc.
On a clear moonlit night, one can see quite well objects illuminated by moonlight. However, the Moon cannot be considered a source of light, because it does not emit, but only reflects the light coming from the Sun.
Is it possible to call a mirror a source of light, with the help of which you start up a "sunbeam"? Explain your answer.
Distinguishing light sources
Rice. 9.2. Powerful Sources artificial light- halogen lamps in the headlights of a modern car
Rice. 9.3. Signals of modern traffic lights are clearly visible even in bright sunshine.
In these traffic lights, incandescent lamps are replaced by LEDs.
Depending on the origin, natural and artificial (man-made) light sources are distinguished.
Natural light sources include the Sun and stars, hot lava and aurora, some living organisms (deep-sea cuttlefish, luminous bacteria, fireflies), etc.
Even in ancient times, people began to create artificial light sources. At first it was bonfires, torches, later - torches, candles, oil and kerosene lamps; in late XIX in. the electric lamp was invented. Today different types electric lamps are used everywhere (Fig. 9.2, 9.3).
What types of electric lamps are used in residential buildings? What lamps are used for multi-colored illumination?
There are also thermal and fluorescent light sources.
Heat sources emit light due to the fact that they have a high temperature (Fig. 9.4).
For the glow of luminescent light sources, a high temperature is not needed: the light radiation can be quite intense, while the source remains relatively cold. Examples of fluorescent light sources are aurora and marine plankton, phone screen, fluorescent lamp, fluorescent road sign, etc.
Rice. 9.4. Some thermal light sources
Studying point and extended light sources
A light source that emits light equally in all directions and whose dimensions, given the distance to the observation point, can be neglected, is called a point light source.
A clear example of point sources of light is the stars: we observe them from the Earth, that is, from a distance that is millions of times greater than the size of the stars themselves.
Light sources that are not point-like are called extended light sources. In most cases, we are dealing with extended light sources. This is a fluorescent lamp, and a mobile phone screen, and a candle flame, and a campfire.
Depending on the conditions, the same light source can be considered both extended and point.
On fig. 9.5 shows a lamp for landscape garden lighting. What do you think, in what case can this lamp be considered a point source of light?
We characterize light receivers
Light receivers are devices that change their properties under the influence of light and with the help of which light radiation can be detected.
Light receivers are artificial and natural. In any light receiver, the energy of light radiation is converted into other types of energy - thermal, which manifests itself in the heating of bodies that absorb light, electrical, chemical and even mechanical. As a result of such transformations, the receivers react in a certain way to light or its change.
For example, some security systems operate on photoelectric light receivers - photocells. Beams of light penetrating the space around the protected object are directed to photocells (Fig. 9.6). If one of these beams is blocked, the photocell will not receive light energy and will immediately “report” this.
In solar panels, photovoltaic cells convert light energy into electrical energy. Many modern solar power plants are large "energy fields" of solar panels.
For a long time, only photochemical light detectors (photographic film, photographic paper) were used to obtain photographs, in which, as a result of the action of light, certain chemical reactions(Fig. 9.7).
From the star closest to us, Alpha Centauri, light travels to Earth for almost 4 years. So, when we look at this star, we actually see what it was like 4 years ago. But there are galaxies that are millions of light years away from us (that is, light travels to them for millions of years!). Imagine that there is a high-tech civilization in such a galaxy. Then it turns out that they see our planet as it was in the time of the dinosaurs!
In modern digital cameras, instead of film, they use a matrix consisting of a large number photocells. Each of these elements receives "its" part of the light flux, converts it into an electrical signal and transmits this signal to a certain place on the screen.
The natural receivers of light are the eyes of living beings (Fig. 9.8). Under the influence of light, certain chemical reactions occur in the retina of the eye, nerve impulses arise, as a result of which the brain forms an idea of the world around us.
Learn about the speed of light
When you look at the starry sky, you can hardly guess that some stars have already gone out. Moreover, several generations of our ancestors admired the same stars, and these stars did not exist even then! How can it be that there is light from a star, but there is no star itself?
The fact is that light propagates in space at a finite speed. The speed c of light propagation is enormous, and in a vacuum it is about three hundred thousand kilometers per second:
Light travels miles of distance in thousandths of a second. That is why, if the distance from the light source to the receiver is small, it seems that the light propagates instantly. But from distant stars, light travels to us for thousands and millions of years.
Summing up
Physical bodies whose atoms and molecules emit light are called light sources. Light sources are thermal and luminescent; natural and artificial; point and extended. For example, the aurora is a naturally extended luminescent light source.
Devices that change their parameters as a result of the action of light and with the help of which light radiation can be detected are called light receivers. In light receivers, the energy of light radiation is converted into other types of energy. The organs of vision of living beings are natural receivers of light.
Light propagates in space at a finite speed. Speed
propagation of light in vacuum is approximately: c = 3 10 m/s. test questions
1. What role does light play in human life? 2. Define a light source. Give examples. 3. Is the moon a source of light? Explain your answer. 4. Give examples of natural and artificial light sources. 5. What do thermal and fluorescent light sources have in common? What is the difference? 6. Under what conditions is a light source considered a point? 7. What devices are called light receivers? Give examples of natural and artificial light receivers. 8. What is the speed of light propagation in vacuum?
Exercise number 9
1. Establish a correspondence between the light source (see figure) and its type.
A Natural thermal B Artificial thermal C Natural luminescent D Artificial luminescent
2. For each line, determine the "extra" word or phrase.
a) candle flame, sun, star, moon, LED lamp;
b) the screen of the switched on computer, lightning, incandescent lamp, torch;
c) fluorescent lamp, gas burner flame, fire, radiolaria.
3. For what approximate time does light travel the distance from the Sun to the Earth - 150 million km?
4. In which of the indicated cases can the Sun be considered a point source of light?
a) observing a solar eclipse;
b) observing the Sun with spaceship flying outside the solar system;
c) determining the time using a sundial.
5. One of the units of length used in astronomy is the light year. How many meters is a light year if it is equal to the distance that light travels in vacuum in one year?
6. Use additional sources of information and find out who and how first measured the speed of light propagation.
This is textbook material.
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- Participant: Maksimova Anna Alekseevna
- Head: Gusarova Irina Viktorovna
Objective - to study light phenomena and the properties of light in experiments, to consider the three main properties of light: straightness of propagation, reflection and refraction of light in media of different density.
Tasks:
- Prepare equipment.
- Carry out the necessary experiments.
- Analyze and present the results.
- Make a conclusion.
Relevance
AT Everyday life we are constantly confronted with light phenomena and their various properties, the work of many modern mechanisms and devices is also associated with the properties of light. Light phenomena have become an integral part of people's lives, so their study is relevant.
The experiments below explain such properties of light as straightness of propagation, reflection and refraction of light.
For providence and description of experiments, the 13th stereotyped edition of A. V. Peryshkin's textbook “Physics. 8th grade." (Drofa, 2010)
Safety
The electrical devices involved in the experiment are fully operational, the voltage on them does not exceed 1.5 V.
The equipment is stably placed on the table, the working order is observed.
At the end of the experiments, electrical appliances are turned off, the equipment is removed.
Experience 1. Rectilinear propagation of light. (p. 149, fig. 120), (p. 149, fig. 121)
Purpose of experience- to prove the rectilinearity of the propagation of light rays in space using a good example.
The rectilinear propagation of light is its property, which we encounter most often. With rectilinear propagation, energy from a light source is directed to any object along straight lines (light rays), without bending around it. This phenomenon can explain the existence of shadows. But in addition to shadows, there are also penumbra, partially illuminated areas. To see under what conditions shadows and penumbras are formed and how light propagates in this case, we will conduct an experiment.
Equipment: an opaque sphere (on a thread), a sheet of paper, a point light source (a flashlight), an opaque sphere (on a thread) smaller in size, for which the light source will not be a point, a sheet of paper, a tripod for fixing the spheres.
Experience progress
Shadow formation
- Let's arrange the objects in the order pocket flashlight-first sphere (fixed on a tripod)-sheet.
- Let's get the shadow displayed on the sheet.
We see that the result of the experiment was a uniform shadow. Suppose that the light propagated in a straight line, then the formation of a shadow can be easily explained: the light coming from a point source along the light beam, touching the extreme points of the sphere, continued to go in a straight line and behind the sphere, which is why the space behind the sphere is not illuminated on the sheet.
Let's assume that the light propagated along curved lines. In this case, the rays of light, bending, would also fall outside the sphere. We would not have seen the shadow, but as a result of the experiment, the shadow appeared.
Now consider the case in which penumbra is formed.
Formation of shade and penumbra
- Let's arrange the objects in the order pocket flashlight-second sphere (fixed on a tripod)-leaf.
- Illuminate the sphere with a flashlight.
- Let's get a shadow, as well as a penumbra, displayed on the sheet.
This time the results of the experiment are shadow and penumbra. How the shadow was formed is already known from the example above. Now, in order to show that the formation of penumbra does not contradict the hypothesis of rectilinear propagation of light, it is necessary to explain this phenomenon.
In this experiment, we took a light source that is not a point, that is, consisting of many points, in relation to a sphere, each of which emits light in all directions. Consider the highest point of the light source and the light beam emanating from it to the lowest point of the sphere. If we observe the movement of the beam behind the sphere to the sheet, then we will notice that it falls on the border of light and penumbra. Rays from similar points going in this direction (from the point of the light source to the opposite point of the illuminated object) create penumbra. But if we consider the direction of the light beam from the above indicated point to the top point of the sphere, then it will be perfectly visible how the beam falls into the penumbra.
From this experience we see that the formation of a penumbra does not contradict the rectilinear propagation of light.
Conclusion
With the help of this experiment, I proved that light propagates in a straight line, the formation of a shadow and penumbra proves the rectilinearity of its propagation.
Phenomenon in life
The straightness of light propagation is widely used in practice. by the most simple example is an ordinary lamp. Also, this property of light is used in all devices that include lasers: laser rangefinders, metal cutting devices, laser pointers.
In nature, the property is found everywhere. For example, light penetrating through gaps in the crown of a tree forms a well-defined straight line passing through the shadow. Of course, if we talk about large scales, it is worth mentioning a solar eclipse, when the moon casts a shadow on the earth, due to which the sun from the earth (of course, we are talking about its shaded area) is not visible. If the light did not propagate in a straight line, this unusual phenomenon would not exist.
Experience 2. Law of reflection of light. (p.154, fig. 129)
Purpose of experience- prove that the angle of incidence of the beam is equal to the angle of its reflection.
Reflection of light is also its most important property. It is thanks to the reflected light, which is captured by the human eye, that we can see any objects.
According to the law of light reflection, the rays, incident and reflected, lie in the same plane with a perpendicular drawn to the interface between two media at the point of incidence of the beam; the angle of incidence is equal to the angle of reflection. Let's check whether these angles are equal, in an experiment, where we take a flat mirror as a reflecting surface.
Equipment: a special device, which is a disk with a printed circular scale, mounted on a stand, in the center of the disk there is a small flat mirror located horizontally (such a device can be made at home using a protractor instead of a disk with a circular scale), the light source is an illuminator attached to the edge of the disc or laser pointer, measurement sheet.
Experience progress
- Let's place the sheet behind the device.
- Turn on the illuminator, directing it to the center of the mirror.
- Let's draw a perpendicular to the mirror to the point of incidence of the beam on the sheet.
- Let us measure the angle of incidence (ﮮα).
- Let us measure the resulting reflection angle (ﮮβ).
- Let's write down the results.
- Let's change the angle of incidence by moving the illuminator, repeat steps 4, 5 and 6.
- Let's compare the results (the value of the angle of incidence with the value of the angle of reflection in each case).
The results of the experiment in the first case:
∠α = 50°
∠β = 50°
∠α = ∠β
In the second case:
∠α = 25°
∠β = 25°
∠α = ∠β
It can be seen from experience that the angle of incidence of a light beam is equal to the angle of its reflection. Light hitting a mirror surface is reflected from it at the same angle.
Conclusion
With the help of experience and measurements, I proved that when light is reflected, the angle of its incidence is equal to the angle of reflection.
Phenomenon in life
We encounter this phenomenon everywhere, as we perceive the light reflected from objects with the eye. A striking visible example in nature is the glare of bright reflected light on water and other surfaces with good reflectivity (the surface absorbs less light than it reflects). Also, one should remember the sunbeams that every child can let out with the help of a mirror. They are nothing more than a ray of light reflected from a mirror.
A person uses the law of reflection of light in such devices as a periscope, a mirror reflector of light (for example, a reflector on bicycles).
By the way, by reflecting light from a mirror, magicians created many illusions, for example, the “Flying Head” illusion. The man was placed in a box among the scenery so that only his head was visible from the box. The walls of the box were covered with mirrors inclined towards the scenery, the reflection from which did not allow the box to be seen and it seemed that there was nothing under the head and it was hanging in the air. The sight is unusual and frightening. Reflection tricks also took place in theaters when a ghost had to be shown on the stage. The mirrors were "fogged" and tilted so that the reflected light from the niche behind the stage was visible in the auditorium. An actor playing a ghost has already appeared in the niche.
Experience 3. Refraction of light.(p. 159, fig. 139)
Purpose of experience- prove that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two media; prove that the angle of incidence of a light beam (≠ 0°) coming from a less dense medium to a denser one is greater than its angle of refraction.
In life, we often meet with the refraction of light. For example, putting a perfectly straight spoon into a transparent glass of water, we see that its image is bent at the border of two media (air and water), although in fact the spoon remains straight.
To better consider this phenomenon, to understand why it occurs and to prove the law of refraction of light (rays, incident and refracted, lie in the same plane with a perpendicular drawn to the interface between two media at the point of incidence of the beam; the ratio of the sine of the angle of incidence to the sine of the angle of refraction is the value is constant for two media) using an example, we will conduct an experiment.
Equipment: two media of different density (air, water), a transparent container for water, a light source (laser pointer), a sheet of paper.
Experience progress
- Pour water into a container, place a sheet behind it at some distance.
- Let us direct a beam of light into water at an angle, ≠ 0°, since at 0° there is no refraction, and the beam passes into another medium unchanged.
- Let us draw a perpendicular to the interface between two media at the point of incidence of the beam.
- Let us measure the angle of incidence of the light beam (∠α).
- Let us measure the angle of refraction of the light beam (∠β).
- Let's compare the angles, make up the ratio of their sines (to find the sines, you can use the Bradis table).
- Let's write down the results.
- Let's change the angle of incidence by moving the light source, repeat steps 4-7.
- Let's compare the values of the sine ratios in both cases.
Let us assume that light rays, passing through media of different densities, experienced refraction. In this case, the angles of incidence and refraction cannot be equal, and the ratios of the sines of these angles are not equal to one. If no refraction has occurred, that is, the light has passed from one medium to another without changing its direction, then these angles will be equal (the ratio of the sines of equal angles is equal to one). To confirm or refute the assumption, consider the results of the experiment.
The results of the experiment in the first case:
∠α = 20
∠β = 15
∠α >∠β
sin∠α = 0.34 = 1.30
sin∠β 0.26
The results of the experiment in the second case:
∠α ˈ= 50
∠β ˈ= 35
∠α ˈ > ∠β ˈ
sin∠α ˈ= 0.77 = 1.35
sin∠β ˈ 0.57
Comparison of sine ratios:
1.30 ~1.35 (due to measurement errors)
sin∠α = sin∠α ˈ = 1.3
sin∠β sin∠β ˈ
According to the results of the experiment, when light is refracted from a less dense medium to a denser one, the angle of incidence is greater than the angle of refraction. the ratios of the sines of the incident and refracted angles are equal (but not equal to one), that is, they are a constant value for the two given media. The direction of the beam when it enters a medium of a different density changes due to a change in the speed of light in the medium. In a denser medium (here, in water), light propagates more slowly, and therefore the angle of passage of light through space changes.
Conclusion
With the help of experience and measurements, I proved that when light is refracted, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for both media, when light rays pass from a less dense medium to a denser one, the angle of incidence is less than the angle of refraction.
Phenomenon in life
We also meet with the refraction of light quite often; one can give many examples of the distortion of the visible image when passing through water and other media. The most interesting example is the occurrence of a mirage in the desert. A mirage occurs when light rays passing from warm layers of air (less dense) to cold layers are refracted, which can often be observed in deserts.
Human refraction of light is used in various devices containing lenses (light is refracted when passing through a lens). For example, in optical instruments such as binoculars, a microscope, a telescope, in cameras. Also, a person changes the direction of light by passing it through a prism, where the light is refracted several times, entering and leaving it.
The objectives of the work have been achieved.
optical phenomena in nature.
Phenomena associated with the reflection of light. The object and its reflection.
The fact that the landscape reflected in the water does not differ from the real one, but is only turned “upside down”, is far from being the case. If a person looks late in the evening at how the lamps are reflected in the water or how the shore descending to the water is reflected, then the reflection will seem shortened to him and will completely “disappear” if the observer is high above the surface of the water. Also, you can never see the reflection of the top of a stone, part of which is immersed in water.
The landscape is seen by the observer as if it were viewed from a point as much deeper than the surface of the water as the observer's eye is above the surface. The difference between the landscape and its image decreases as the eye approaches the surface of the water, as well as as the object moves away.
Rainbow. 
The rainbow is a beautiful celestial phenomenon that has always attracted the attention of man.
The rainbow theory was first given in 1637 by René Descartes. He explained the rainbow as a phenomenon associated with the reflection and refraction of light in raindrops.
The rainbow is observed in the direction opposite to the Sun, against the background of rain clouds or rain. A multi-colored arc is usually located at a distance of 1-2 km from the observer, and sometimes it can be observed at a distance of 2-3 m against the background of water drops formed by fountains or water sprayers.
The rainbow has seven primary colors that smoothly transition from one to another. The shape of the arc, the brightness of the colors, the width of the stripes depend on the size of the water droplets and their number. Large drops create a narrower rainbow, with sharply prominent colors, small drops create an arc that is blurry, faded and even white. That is why a bright narrow rainbow is visible in the summer after a thunderstorm, during which large drops fall.
Most often we see one rainbow. It is not uncommon for two rainbow stripes to appear simultaneously in the sky, located one after the other; an even greater number of celestial arcs are observed - three, four and even five at the same time.
Polar Lights.
One of the most beautiful optical phenomena of nature is the aurora borealis. In most cases, the auroras are green or blue-green in color, with occasional patches or borders of pink or red. Auroras are observed in two main forms - in the form of ribbons and in the form of spots.
According to the brightness of the aurora, they are divided into four classes, which differ from each other by an order of magnitude. The 1st class includes auroras that are barely noticeable and approximately equal in brightness to the Milky Way, while the 4th class radiance illuminates the Earth as brightly as the full moon.
light beam in geometric optics, a line along which light energy is transferred. Less clearly, but more clearly, a beam of light of small transverse size can be called a light beam.
The concept of a light beam is a cornerstone approximation of geometric optics. This definition implies that the direction of the flow of radiant energy (the path of the light beam) does not depend on the transverse dimensions of the light beam. Due to the fact that light is a wave phenomenon, diffraction takes place, and as a result, a narrow beam of light does not propagate in any one direction, but has a finite angular distribution.
The law of rectilinear propagation of light: In a transparent homogeneous medium, light travels in straight lines.
In connection with the law of rectilinear propagation of light, the concept of a light beam appeared, which has a geometric meaning as a line along which light propagates. Light beams of finite width have a real physical meaning. The light beam can be considered as the axis of the light beam. Since light, like any radiation, carries energy, we can say that a light beam indicates the direction of energy transfer by a light beam. Also, the law of rectilinear propagation of light makes it possible to explain how solar and lunar eclipses occur (The figure shows solar eclipse. At lunar eclipse The Moon and the Earth "swap" places).
Light dispersion(light decomposition) is the phenomenon of the dependence of the absolute refractive index of a substance on the wavelength (or frequency) of light (frequency dispersion), or, equivalently, the dependence of the phase velocity of light in a substance on the wavelength (or frequency). Experimentally discovered by Newton around 1672, although theoretically well explained much later.
Colour- a qualitative subjective characteristic of electromagnetic radiation in the optical range, determined on the basis of the emerging physiological visual sensation and depending on a number of physical, physiological and psychological factors.
The sensation of color occurs in the brain during excitation and inhibition of color-sensitive cells - receptors in the human or other animal's eye retina, cones. It is believed (although to date it has not been proven by anyone) that in humans and primates there are three types of cones that differ in spectral sensitivity - conditionally “red”, conditionally “green” and conditionally “blue”. The light sensitivity of cones is not high, therefore, sufficient illumination or brightness is necessary for good color perception. The most rich in color receptors are the central parts of the retina.
Each color sensation in a person can be represented as the sum of the sensations of these three colors (the so-called "three-component theory of color vision"). It has been established that reptiles, birds and some fish have a wider area of perceived optical radiation. They perceive near ultraviolet (300-380 nm), blue, green and red part of the spectrum. When the brightness necessary for color perception is reached, the most highly sensitive receptors of twilight vision - rods - are automatically turned off.
Reflection- the phenomenon of partial or complete return of waves (electromagnetic), reaching the interface between two media (obstacles), to the environment from which they approach this boundary.
Law of light reflection- sets a change in the direction of the light beam as a result of a meeting with a reflecting (mirror) surface: the incident and reflected rays lie in the same plane with the normal to the reflecting surface at the point of incidence, and this normal divides the angle between the rays into two equal parts. The widely used but less accurate formulation "angle of reflection equals angle of incidence" does not indicate the exact direction of reflection of the beam.
The universal concept in physics is the speed of light. c. Its value in vacuum represents not only the limiting speed of propagation of electromagnetic oscillations of any frequency, but also in general the limiting speed of propagation of any impact on material objects. When light propagates in various media, the speed of light v decreases: v=c/n, where n is the refractive index of the medium, which characterizes its optical properties and depends on the frequency of light n = n(v).
Refraction- a change in the direction of propagation of waves of electromagnetic radiation that occurs at the interface between two media transparent to these waves or in the thickness of a medium with continuously changing properties.
The refraction of light at the boundary of two media gives a paradoxical visual effect: straight objects crossing the interface in a denser medium appear to form a larger angle with the normal to the interface (that is, refracted "up"); while a ray entering a denser medium propagates in it at a smaller angle to the normal (that is, it is refracted "down"). The same optical effect leads to errors in the visual determination of the depth of the reservoir, which always seems to be less than it actually is.
The refraction of light in the Earth's atmosphere leads to the fact that we observe the sunrise a little earlier, and the sunset a little longer than it would be in the absence of an atmosphere. For the same reason, near the horizon, the disk of the Sun looks noticeably flattened along the vertical.
Snell's law refraction of light describes the refraction of light at the boundary of two media. It can also be used to describe the refraction of waves of a different nature, such as sound waves.
The angle of incidence of light on the surface is related to the angle of refraction by the relation
Here:
n 1 is the refractive index of the medium from which light is incident on the interface;
A 1- angle of incidence of light - the angle between the beam incident on the surface and the normal to the surface;
n 2 is the refractive index of the medium into which light enters after passing through the interface;
A2- angle of refraction of light - the angle between the beam passing through the surface and the normal to the surface.
Lens- a part of an optically transparent homogeneous material, limited by two polished refractive surfaces of revolution, for example, spherical or flat and spherical. Currently, "aspherical lenses" are increasingly being used, the shape of the surface of which differs from the sphere. As the lens material, optical materials such as glass, optical glass, optically transparent plastics, and other materials are commonly used.
Depending on the shape, there are converging (positive) and diverging (negative) lenses. The group of converging lenses usually includes lenses, in which the middle is thicker than their edges, and the group of diverging lenses is lenses, the edges of which are thicker than the middle. It should be noted that this is true only if the refractive index of the lens material is greater than that of environment. If the refractive index of the lens is less, the situation will be reversed. For example, an air bubble in water is a biconvex diffusing lens.
Lenses are characterized, as a rule, by their optical power (measured in diopters), or focal length.
If light from a very distant source falls on the lens, the rays of which can be represented as going in a parallel beam, then when they leave it, the rays will refract at a large angle, and point F, the point of intersection of these rays, will move on the optical axis closer to the lens. Under these conditions, the point of intersection of the rays emerging from the lens is called the focus. F, and the distance from the center of the lens to the focus is focal length.
optical power- value characterizing the refractive power of axisymmetric lenses and centered optical systems of such lenses. The optical power is measured in diopters(in the SI system) and inversely proportional to the focal length:
Imaging, which gives a thin lens.
Let us consider a ray SA of an arbitrary direction, incident on the lens at point A. Let us construct the line of its propagation after refraction in the lens. To do this, we construct a beam OB parallel to SA and passing through the optical center O of the lens. According to the first property of the lens, the beam OB will not change its direction and intersect the focal plane at point B. According to the second property of the lens, the beam SA parallel to it, after refraction, must intersect the focal plane at the same point. Thus, after passing through the lens, the beam SA will follow the path AB.
Other rays can be constructed in a similar way, for example, the ray SPQ.
Let us denote the distance SO from the lens to the light source as u, the distance OD from the lens to the focusing point of the rays as v, the focal length OF as f. Let us derive a formula relating these quantities.
Consider two pairs similar triangles: 1) SOA and OFB; 2) DOA and DFB. Let's write down the proportions
Dividing the first ratio by the second, we get
After dividing both parts of the expression by v and rearranging the terms, we arrive at the final formula
Photometry. The power of light and illumination.
Photometry- common for all sections of applied optics scientific discipline, on the basis of which quantitative measurements of the energy characteristics of the radiation field are made.
The power of light- this is the quantitative value of the radiation flux per unit solid angle of the limit of its propagation. In other words, this is the amount of light (in lumens) per 1 steradian.
The solid angle must be chosen in such a way that the flow limited by it can be considered the most uniform. Then the unit of the solid angle in this direction from the source will contain the luminous intensity numerically equal to the luminous flux
SI unit: candela (cd) = lumen (lm) / steradian (sr)
illumination- a physical quantity numerically equal to the luminous flux incident on a unit surface:
The SI unit for measuring illumination is the lux (1 lux = 1 lumen/m2).
Light flow- a physical quantity that characterizes the "amount" of light energy in the corresponding radiation flux. In other words, this is the power of such radiation, which is available for perception by the normal human eye (F).
Eye- a sensory organ of humans and animals that has the ability to perceive electromagnetic radiation in the light wavelength range and provides the function of vision. 90 percent of the information from the outside world comes through the eye.
Myopic is called such an eye, in which the focus in a calm state of the eye muscle lies inside the eye. Nearsightedness may be due to the distance between the retina and the lens compared to the normal eye. If an object is located at a distance of 25 cm from the myopic eye, then the image of the object will not be on the retina, but closer to the lens, in front of the retina. In order for the image to appear on the retina, you need to bring the object closer to the eye. Therefore, in a near-sighted eye, the distance of best vision is less than 25 cm. An eye is called far-sighted, in which the focus lies behind the retina in a calm state of the eye muscle. Farsightedness can also be due to the fact that the retina is located closer to the lens compared to the normal eye and the image of the object is obtained behind the retina of such an eye. If the object is removed from the eye, then the image will fall on the retina, hence the name of this defect - farsightedness.
Nearsightedness and farsightedness are corrected by lenses. The invention of glasses was a great boon for people with visual impairments.
In a myopic eye, the image is produced inside the eye in front of the retina. In order for it to move to the retina, it is necessary to reduce the optical power of the refractive system of the eye. For this, a diverging lens is used.
The optical power of the far-sighted eye system, on the contrary, must be increased in order for the image to fall on the retina. For this, a converging lens is used.
Optical devices.
Optical devices- devices in which the radiation of any region of the spectrum (ultraviolet, visible, infrared) is converted (transmitted, reflected, refracted, polarized). They can increase, decrease, improve (in rare cases worsen) the quality of the image, make it possible to see the desired object indirectly.
The term "optical devices" is a special case of the more general concept of optical systems, which also includes biological organs capable of converting light waves.
Visual (spyglass)- an optical device for observing distant objects, consists of a lens that creates actual image objects, and an eyepiece to magnify that image.
Microscope- a device designed to obtain enlarged images, as well as to measure objects or structural details that are invisible to the naked eye. It is a collection of lenses.
magnifying glass- an optical system consisting of a lens or several lenses, designed to magnify and observe small objects located at a finite distance.
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abstract
On the topic: light phenomena
Completed by: Khrapatov D. A.
Checked:
1. Light. Sources of light
2. Spread of light
3. Light reflection
4. Flat mirror
5. Mirror and diffuse image
6. Refraction of light
8. Images given by the lens
Light. Sources of light
Light ... its significance in our life is very great. It is difficult to imagine life without light. After all, all living things are born and develop under the influence of light and heat.
Human activity in the initial periods of its existence - obtaining food, protection from enemies, hunting - was dependent on daylight. Then a person learned to make and maintain fire, began to illuminate his dwelling, to hunt with torches. But in all cases, his activity could not proceed without illumination.
The light sent by celestial bodies made it possible to determine the location and movement of the Sun, stars, planets, the Moon and other satellites. The study of light phenomena helped to create devices with which they learned about the structure and even composition celestial bodies located at a distance of many billions of kilometers from the Earth. Telescope observations and photographs of the planets were used to study their cloud cover, surface features, and rotation speeds. We can say that the science of astronomy arose and developed thanks to light and vision.
The creation of artificial lighting, so necessary for man, is based on the study of light. Light is needed everywhere: traffic safety is associated with the use of headlights, road lighting; in military equipment lighting rockets, searchlights are used; normal lighting of the workplace contributes to increased productivity; sunlight increases the body's resistance to disease, improves a person's mood.
What is light? Why and how do we perceive it?
The branch of science devoted to the study of light is also called optics (from the Greek optos - visible, visible).
Light (optical) radiation is produced by light sources.
There are natural and artificial light sources. Natural light sources include the sun, stars, aurora, lightning; to artificial - lamps, candles, TV and others.
We see the source of light because the radiation created by the name falls into our eyes. But we also see bodies that are not sources of light - trees, houses, the walls of a room, the moon, planets, and so on. However, we only see them when they are illuminated by light sources. The radiation coming from light sources, falling on the surface of objects, changes its direction and enters the eyes.
2. Spread of light
Optics is one of the oldest sciences.
Long before they knew what light was, some of its properties were discovered and used in practice.
Based on observations and experiments, the laws of light propagation were established, using the concept of a ray of light.
A BEAM is a line along which light propagates.
The law of rectilinear propagation of light.
Light in a transparent homogeneous medium propagates in straight lines.
For this law, we can consider an example - the formation of a shadow:
If we want to prevent the light from the lamp from entering our eyes, we can block it with our hand or put a lampshade on the lamp. If the light did not travel in straight lines, then it could go around the edges of the obstacle and get into our eyes. For example, it is impossible to “block” the sound from the hand, it will go around this obstacle and we will hear it.
Let's take a look at this phenomenon.
Take a light bulb from a pocket torch. Let's place the screen at some distance from it. The lamp illuminates the screen completely. Let's place an opaque body (for example, a metal ball) between the light bulb and the screen. Now a dark circle will appear on the screen, as a shadow has formed behind the ball - a space into which light from the source does not fall.
But a clearly described shadow, which is obtained in the described experience, we do not always see in life. If the dimensions of the light source are much larger, then a penumbra will form around the shadow. If our eye were in the shadow area, then we would not see the light source, but from the penumbra area, we would see one of its edges. The law of propagation of light was used by the ancient Egyptians in order to install columns, pillars, walls in a straight line. They arranged the columns in such a way that because of the column closest to the eye, all the others were not visible.
3. Light reflection
Let's direct a beam of light from the light source to the screen. The screen will be illuminated, but we will not see anything between the source and the screen. If a piece of paper is placed between the source and the screen, then it will be visible. This happens because the radiation, having reached the surface of the sheet, is reflected, changes its direction and enters our eyes. The entire beam of light becomes visible if the air between the screen and the light source is dusted. In this case, the dust particles reflect light and direct it into the eyes of the observer.
Law of light reflection:
The incident and reflected rays lie in the same plane with the perpendicular to the reflecting surface, raised at the point of incidence of the ray.
Let the straight line MN be the surface of the mirror, AO the incident and OB the reflected rays, OS the perpendicular to the mirror surface at the point of incidence of the ray.
The angle formed by the incident beam AO and the perpendicular OS (the angle of AOC) is called the angle of incidence. It is denoted by the letter α ("alpha"). The angle formed by the reflected beam OB and the same perpendicular to the OS (i.e., the angle of the COB) is called the angle of reflection, it is denoted by the letter β ("beta").
By moving the light source along the edge of the disk, we change the angle of incidence of the beam. Let's repeat the experiment, but now each time we will note the angle of incidence and the corresponding angle of reflection.
Observations and measurements show that for all values of the angle of incidence, equality between it and the angle of reflection is preserved.
So, the second law of light reflection says: the angle of reflection is equal to the angle of incidence.
4. Flat mirror
A mirror whose surface is a plane is called a flat mirror.
When an object is in front of a mirror, it seems that the same object is behind the mirror, what we see behind the mirror is called the image of the object.
To begin with, let's explain how the eye perceives the object itself, for example, a candle. Rays of light radiate from each point of the slash in all directions. Some of them enter the eye in a diverging beam. The eye sees (perceives) a point in the place where the rays come from, i.e. where they intersect, which is not really a point.
When we look in a mirror, we see a virtual image of our own face.
Let's place a piece of flat glass vertically - it will serve as a mirror. But since glass is transparent, we can also see what is behind it. Place a lit candle in front of the glass. In the glass we will see her image. On the other side of the glass (where we see the image) we will put the same, but unlit candle and move it until it seems to be lit. This will mean that the image of a lit candle is located where an unlit candle stands.
Let's measure the distance from the candle to the glass and from the glass to the image of the candle. These distances will be the same.
Experience also shows that the height of the candle image is equal to the height of the candle itself, i.e. the dimensions of the image in a flat mirror are equal to the dimensions of the object.
So, experience shows that the image of an object in a flat mirror has the following features: this image is imaginary, direct, equal in size to the object, it is located at the same distance behind the mirror as the object is located in front of the mirror.
The image in a flat mirror has another feature. Look at the image of your right hand in a flat mirror, the fingers in the image are positioned as if it were a left hand.
5. Mirror and diffuse image
In a flat mirror, we see an image that differs little from the object itself. This is due to the fact that the surface of the mirror is flat and smooth, and the fact that the mirror reflects most of the light falling on it (from 70 to 90%).
The mirror surface reflects the beam of light incident on it in a directed way. Let, for example, a beam of parallel rays from the sun fall on a mirror. Rays are also reflected by a parallel beam.
Any non-mirror, i.e. a rough, non-smooth surface scatters light: it reflects a beam of parallel rays falling on it in all directions. This is explained by the fact that a rough surface consists of a large number of very small flat surfaces arranged randomly at different angles to each other. Each small flat surface reflects light in a certain direction. But all together they direct the reflected rays in different directions, i.e. scatter light in different directions.
6. Refraction of light
A spoon or pencil dipped into a glass of water seems to be broken at the border between water and air. This can only be explained by the fact that the rays of light coming from the spoon have a different direction in water than in air.
Changing the direction of light propagation when it passes through the boundary of two media is called light refraction.
When a beam passes from glass (water) to air, the angle of refraction is greater than the angle of incidence.
The ability to refract rays in different media is different. For example, a diamond refracts light rays more strongly than water or glass.
If a beam of light falls on the surface of a diamond at an angle of 60 *, then the angle of refraction of the beam is approximately 21 *. At the same angle of incidence of the beam on the surface of the water, the angle of refraction is about 30*.
When a beam passes from one medium to another, light is refracted according to the following provisions:
1. The incident and refracted rays lie in the same plane with a perpendicular drawn at the point of incidence of the beam to the plane of separation of two media.
2. Depending on which medium the beam passes into, the angle of refraction may be less or greater than the angle of incidence.
7. Lenses
Reflection and refraction of light is used to change the direction of rays or, as they say, to control light beams. This is the basis for the creation of special optical instruments, such as a searchlight, a magnifying glass, a microscope, a camera, and others. The main part of most of them is the lens.
In optics, spherical lenses are most commonly used. Such lenses are bodies made of optical or organic glass, bounded by two spherical surfaces.
Lenses are different, limited on one side by a spherical surface and on the other by a flat surface, or concave-convex, but the most commonly used are convex and concave.
A convex lens converts a beam of parallel rays into a converging one, collects it into one point. Therefore, a convex lens is called a converging lens.
A concave lens converts a beam of parallel rays into a divergent one. Therefore, a concave lens is called a diverging lens.
We have considered lenses bounded by spherical surfaces on both sides. But lenses are also made and used, limited on one side by a spherical surface, and on the other side by a flat surface, or concave-convex lenses. However, despite this, lenses are either converging or diverging. If the middle part of the lens is thicker than its edges, then it collects rays, and if it is thinner, then it scatters.
8. Images given by the lens
The lens controls the light rays. However, with the help of a lens, it is possible not only to collect and scatter light rays, but also to obtain a variety of images of objects. It is thanks to this ability of lenses that they are widely used in practice. So the lens in a movie camera gives a magnification of hundreds of times, and in a camera the lens also gives a reduced image of the object being photographed.
1. If an object is between the lens and its focus, then its image is enlarged, imaginary, direct, and it is located farther from the lens than the object.
Such an image is obtained when using a magnifying glass when assembling watches, reading small text, etc.
2. If the object is between the focus and the double focus of the lens, then the lens gives it an enlarged, inverted, real image; it is located on the other side of the lens in relation to the subject, behind double the focal length.
Such an image is used in a projection apparatus, in a movie camera.
3. The object is behind double the distance of the lens.
In this case, the lens gives a reduced, inverted, real image of the object, lying on the other side of the lens between its Fox and double focus.
Such an image is used in photographic equipment.
A lens with more convex surfaces refracts rays more than a lens with less curvature. Therefore, the focal length of a more convex lens is less than that of a less convex lens. A lens with a shorter focal length produces more magnification than a long focal length lens.
The magnification of the subject will be the greater, the closer the subject is to the focus. Therefore, with the help of lenses it is possible to obtain images with high and very high magnification. In the same way, images with different reductions can be obtained.
Literature
1. Light. Sources of light.
2. Myopia and hyperopia. Glasses.
3. Light. Edited by N.A. motherland
Rainbow; the shadow cast by the object; blue sky; the multicolor of the world around us - these are just a few examples of light phenomena. These phenomena are studied in the section of physics called "optics" (from the Greek optike - the science of visual perception).
Light sources are familiar to you. They can be divided into natural (Sun, stars) and artificial (electric lamps).
An important property of light is the straightness of its propagation. Only under this condition is the formation of a shadow and the occurrence of eclipses of the Sun and Moon possible.
Rays of light are reflected from obstacles. If the rays fall on a mirror, they are reflected in such a way that we see a life-size object in the mirror. If rays of light fall on an uneven surface, they are reflected in all directions and make this surface illuminated. That is why we can see objects that do not themselves glow (including such celestial bodies as planets and their satellites).
When rays of light enter from the air into some other transparent medium (water, glass), they are refracted (look at the side of a spoon in a glass of water and you will see that at the air-water interface, the spoon "fractures").
If white light falls on a trihedral glass prism, it is refracted and simultaneously decomposed into seven colors. This is the phenomenon of dispersion. The colors are always arranged in a certain order: red, orange, yellow, green, blue, indigo, violet. (Tick, can words be colored?) Such a colored band is called a spectrum. The sequence of colors in the spectrum can be remembered with a simple phrase: "Every hunter wants to know where the pheasant sits." Dispersion is also observed in nature. Remember the rainbow. It is obtained due to the fact that sunlight is refracted in raindrops, as in prisms.
But what is light? It took scientists a long time to find the answer to this question. And the answer was unexpected. The fact is that in some phenomena, light behaves like a stream of particles (they are called light quanta, or photons), in others - like a wave. For example, the iridescence of CD-ROM discs is due to the fact that light exhibits wave properties, and the deviation of comet tails from the Sun is due to light pressure associated with the idea of light as a stream of particles.
It is impossible to overestimate the importance of light for the knowledge of the world around us. After all, we receive the greatest part of information about it thanks to light. The study of the light coming to us from celestial bodies allows us to learn a lot about them. Here the spectra of celestial bodies play a particularly important role. This is a kind of their "passport", deciphering which astronomers obtain information about temperature, chemical composition celestial bodies, the speeds with which they move, approaching us or moving away from us, and much more. In everyday life, we meet with various optical devices - from glasses to telescopes. They, of course, could not have been created without the study of light phenomena.