In a physics video lesson from the Academy of Entertaining Sciences, Professor Daniil Edisonovich continues the conversation about light begun in the previous episode of the program. TV viewers already know what reflection of light is, but what is refraction of light? It is the refraction of light that explains some of the strange optical phenomena that we can observe in our everyday life.
The phenomenon of light refraction
Why do the legs of people standing in the water seem shorter than they really are, and if you look at the bottom of the river, it seems closer? It's all about the phenomenon of light refraction. Light always tries to move in a straight line, the shortest path. But getting from one physical environment to another part of the sun's rays changes direction. In this case, we are dealing with the phenomenon of light refraction. This is why a spoon in a glass of tea appears broken - the light from the part of the spoon that is in the tea reaches our eyes at a different angle than the light from the part of the spoon that is above the surface of the liquid. In this case, the refraction of light occurs at the boundary of air and water. When reflected, a ray of light travels the shortest path, and when refracted, it moves the fastest. Using the laws of reflection and refraction of light, people have created many things without which our life today is unthinkable. Telescopes, periscopes, microscopes, magnifying glasses, all this would be impossible to create without knowledge of the laws of refraction and reflection of light. A magnifying glass magnifies because, having passed through it, rays of light enter the eye at an angle greater than the rays reflected from the object itself. To do this, the object must be placed between the magnifying glass and its optical focus. Optical focus; this is the point at which initially parallel rays intersect (focus) after passing through a collecting system (or where their extensions intersect if the system is scattering). A lens (such as an eyeglass lens) has two sides, so a ray of light is refracted twice—as it enters and exits the lens. The surface of the lens can be curved, concave or flat, which determines exactly how the phenomenon of light refraction will occur in it. If both sides of a lens are convex, it is a converging lens. Refracted in such a lens, light rays are concentrated at one point. This is called the main focus of the lens. A lens with concave sides is called a diverging lens. At first glance, it lacks focus, because the rays passing through it are scattered and diverge to the sides. But if we redirect these rays back, then they, again passing through the lens, will gather at a point, which will be the focus of this lens. There is a lens in the human eye, it is called the lens. It can be compared to a film projector, which projects an image onto a screen - the back wall of the eye (retina). So it turns out that the lake is a giant lens that causes the phenomenon of light refraction. That’s why the legs of the fishermen standing in it seem short. Rainbows also appear in the sky due to lenses. Their role is played by tiny droplets of water or snow particles. Rainbows occur when sunlight is refracted and reflected by droplets of water (rain or fog) floating in the atmosphere. These droplets bend light of different colors differently. As a result, white light is decomposed into a spectrum (light dispersion occurs). An observer who stands with his back to the light source sees a multi-colored glow that emanates from space in circles (arcs).
Processes that are associated with light are an important component of physics and surround us everywhere in our everyday lives. The most important in this situation are the laws of reflection and refraction of light, on which modern optics is based. The refraction of light is an important part of modern science.
Distortion effect
This article will tell you what the phenomenon of light refraction is, as well as what the law of refraction looks like and what follows from it.
Basics of a physical phenomenon
When a beam falls on a surface that is separated by two transparent substances that have different optical densities (for example, different glasses or in water), some of the rays will be reflected, and some will penetrate into the second structure (for example, they will propagate in water or glass). When moving from one medium to another, a ray typically changes its direction. This is the phenomenon of light refraction.
The reflection and refraction of light is especially visible in water.
Distortion effect in water
Looking at things in water, they appear distorted. This is especially noticeable at the boundary between air and water. Visually, underwater objects appear to be slightly deflected. The described physical phenomenon is precisely the reason why all objects appear distorted in water. When the rays hit the glass, this effect is less noticeable.
Refraction of light is a physical phenomenon that is characterized by a change in the direction of movement of a solar ray at the moment it moves from one medium (structure) to another.
To improve our understanding of this process, consider an example of a beam hitting water from air (similarly for glass). By drawing a perpendicular line along the interface, the angle of refraction and return of the light beam can be measured. This index (angle of refraction) will change as the flow penetrates the water (inside the glass).
Note! This parameter is understood as the angle formed by a perpendicular drawn to the separation of two substances when a beam penetrates from the first structure to the second.
Beam Passage
The same indicator is typical for other environments. It has been established that this indicator depends on the density of the substance. If the beam falls from a less dense to a denser structure, then the angle of distortion created will be greater. And if it’s the other way around, then it’s less.
At the same time, a change in the slope of the decline will also affect this indicator. But the relationship between them does not remain constant. At the same time, the ratio of their sines will remain a constant value, which is reflected by the following formula: sinα / sinγ = n, where:
- n is a constant value that is described for each specific substance (air, glass, water, etc.). Therefore, what this value will be can be determined using special tables;
- α – angle of incidence;
- γ – angle of refraction.
To determine this physical phenomenon, the law of refraction was created.
Physical law
The law of refraction of light fluxes allows us to determine the characteristics of transparent substances. The law itself consists of two provisions:
- First part. The beam (incident, modified) and the perpendicular, which was restored at the point of incidence on the boundary, for example, of air and water (glass, etc.), will be located in the same plane;
- The second part. The ratio of the sine of the angle of incidence to the sine of the same angle formed when crossing the boundary will be a constant value.
Description of the law
In this case, at the moment the beam exits the second structure into the first (for example, when the light flux passes from the air, through the glass and back into the air), a distortion effect will also occur.
An important parameter for different objects
The main indicator in this situation is the ratio of the sine of the angle of incidence to a similar parameter, but for distortion. As follows from the law described above, this indicator is a constant value.
Moreover, when the value of the decline slope changes, the same situation will be typical for a similar indicator. This parameter is of great importance because it is an integral characteristic of transparent substances.
Indicators for different objects
Thanks to this parameter, you can quite effectively distinguish between types of glass, as well as various precious stones. It is also important for determining the speed of light in various environments.
Note! The highest speed of light flow is in a vacuum.
When moving from one substance to another, its speed will decrease. For example, in diamond, which has the highest refractive index, the speed of photon propagation will be 2.42 times higher than that of air. In water, they will spread 1.33 times slower. For different types of glass, this parameter ranges from 1.4 to 2.2.
Note! Some glasses have a refractive index of 2.2, which is very close to diamond (2.4). Therefore, it is not always possible to distinguish a piece of glass from a real diamond.
Optical density of substances
Light can penetrate through different substances, which are characterized by different optical densities. As we said earlier, using this law you can determine the density characteristic of the medium (structure). The denser it is, the slower the speed at which light will propagate through it. For example, glass or water will be more optically dense than air.
In addition to the fact that this parameter is a constant value, it also reflects the ratio of the speed of light in two substances. The physical meaning can be displayed as the following formula:
This indicator tells how the speed of propagation of photons changes when moving from one substance to another.
Another important indicator
When a light flux moves through transparent objects, its polarization is possible. It is observed during the passage of a light flux from dielectric isotropic media. Polarization occurs when photons pass through glass.
Polarization effect
Partial polarization is observed when the angle of incidence of the light flux at the boundary of two dielectrics differs from zero. The degree of polarization depends on what the angles of incidence were (Brewster's law).
Full internal reflection
Concluding our short excursion, it is still necessary to consider such an effect as full internal reflection.
The phenomenon of full display
For this effect to appear, it is necessary to increase the angle of incidence of the light flux at the moment of its transition from a more dense to a less dense medium at the interface between substances. In a situation where this parameter exceeds a certain limiting value, then photons incident on the boundary of this section will be completely reflected. Actually, this will be our desired phenomenon. Without it, it was impossible to make fiber optics.
Conclusion
The practical application of the behavior of light flux has given a lot, creating a variety of technical devices to improve our lives. At the same time, light has not yet revealed all its possibilities to humanity and its practical potential has not yet been fully realized.
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1. We conduct experiments on the refraction of light
Let's conduct such an experiment. Let us direct a narrow beam of light to the surface of water in a wide vessel at a certain angle to the surface. We will notice that at the points of incidence the rays are not only reflected from the surface of the water, but also partially pass into the water, changing their direction (Fig. 3.33).
- The change in the direction of propagation of light when it passes through the interface between two media is called refraction of light.
The first mention of the refraction of light can be found in the works of the ancient Greek philosopher Aristotle, who wondered: why does a stick appear broken in water? And in one of the ancient Greek treatises the following experiment is described: “You need to stand so that the flat ring placed on the bottom of the vessel is hidden behind its edge. Then, without changing the position of the eyes, pour water into the vessel.
Rice. 3.33 Scheme of an experiment to demonstrate the refraction of light. Passing from air to water, a ray of light changes its direction, shifting towards the perpendicular established at the point of incidence of the ray
2. There are the following relationships between the angle of incidence and the angle of refraction:
a) if the angle of incidence increases, the angle of refraction also increases;
b) if a ray of light passes from a medium with a lower optical density to a medium with a higher optical density, then the angle of refraction will be less than the angle of incidence;
c) if a ray of light passes from a medium with a higher optical density to a medium with a lower optical density, then the angle of refraction will be greater than the angle of incidence.
(It should be noted that in high school, after taking a trigonometry course, you will become more familiar with the refraction of light and learn about it at the level of laws.)
4. We explain some optical phenomena by the refraction of light
When we, standing on the shore of a reservoir, try to determine its depth by eye, it always seems smaller than it actually is. This phenomenon is explained by the refraction of light (Fig. 3.37).
Rice. 3. 39. Optical devices whose operation is based on the phenomenon of light refraction
- Test questions
1. What phenomenon do we observe when light passes through the interface between two media?
L.I. Mandelstam studied the propagation of electromagnetic waves, primarily visible light. He discovered a number of effects, some of which now bear his name (Raman scattering, the Mandelstam-Brillouin effect, etc.).
Option 1. Equipment: a device for studying the laws of geometric optics, a rectifier VS-24 or VS 4-12, a flat mirror made from parts of the device.
When preparing a geometric optics device for operation, adjust the screen illumination. To do this, loosen the ball joint and rotate or move the illuminator until the middle strip of light passes through the entire screen (along its diameter). The illuminator is fixed in this position. If at the same time the strip of light is blurry and not sharp, then by releasing the screw fixing the electric cartridge in the illuminator, rotate, lower or raise the electric cartridge until a clear strip of light is obtained on the screen. If the side strips of light do not reach the edge of the screen, then the tilt of the illuminator should be changed. After adjustment, all screws are securely fastened.
The installation is assembled according to Figure 278. Using a clamp, a flat mirror is installed from a set of optical parts so that its reflective surface coincides with the horizontal axis. Only one middle ray is left. They change the angle of incidence from 0 to 90°, note the angle of reflection, compare these angles, and draw a conclusion.
The experiment is repeated, demonstrating the reversibility properties of light beams, for which the illuminator is transferred from one part of the disk to another. (When demonstrating experiments in geometric optics, the room should be darkened.)
Rice. 278 Fig. 280
Experiment 2. Refractions of light
Option 1. Equipment:
A transparent half-cylinder is placed on the screen with the matte side facing the screen and the flat cut up so that it coincides with the horizontal axis. The center of the half-cylinder is aligned with the center of the screen using marks on the matte surface of the half-cylinder (Fig. 280).
When demonstrating the experiment, use the middle beam. The beam is directed to the center of the half-cylinder perpendicular to the plane (the beam passes without changing direction). Deflect the incident ray from the perpendicular and notice that the refracted ray exits the half-cylinder at a different angle. The angles of incidence and refraction are compared and a conclusion is drawn.
Repeat the experiment at a different angle of incidence. (During the experiment, you should pay attention to the bifurcation of the light beam at the interface between the two media.)
Experiment 3. The phenomenon of total reflection of light
Option 1. Equipment: a device for studying the laws of geometric optics, rectifier VS-24 or VS 4-12, half-cylinder from a set of optical parts.
Having paid attention to the ratio of the angles of incidence and refraction in the previous experiment (Fig. 280), the position of the half-cylinder is changed. Its convex side is installed towards the illuminator (the flat cut coincides with the horizontal axis). The angles of incidence are changed, compared with the angles of refraction, and a conclusion is drawn.
The ratio of the angles of incidence and refraction is compared depending on the ratio of the optical density of the media (the results of this and previous experiments). They draw a conclusion.
Make sure that as the angle of incidence increases, the brightness of the reflected beam increases, and that of the refracted beam decreases. Increase the angle of incidence until the refracted beam disappears. With a further increase in the angle of incidence, only the reflected beam will be observed. The phenomenon of total reflection of light is observed.
Question. What is the limiting angle of total reflection? (Give your answer to one significant figure.)
Option 2. Equipment: projection device, aquarium.
The installation is assembled according to Figure 281. A layer of water 7-8 cm thick is poured into a glass bath (aquarium) and tinted with pine concentrate. A horizontal slit is installed in front of the condenser of the projection apparatus, and a flat mirror is placed on the lens frame. A beam of light is directed onto the side wall of the glass bath. The refraction of a light beam in water, total reflection from the surface of the water, and refraction when the beam exits the bath are observed. By changing the angle of incidence, one can observe multiple total reflections of the light beam from the surface of the water and the bottom of the bath.
