Optics: physics, grade 8. Law of reflection: formula

Today we will talk about the law of reflection of light. We also highlight the part of linear optics to which this phenomenon belongs.

School and light

Children go to first grade with impatience. They are interested in what it means to study, they are captivated by fuss with textbooks and notebooks. But discipline is a strict thing. And the psychological laws of a closed group of children are quite cruel. Therefore, older students associate with school only reluctance to go there. Nevertheless, with a creative approach to knowledge itself, one can change the view of the world of lessons and diaries. Today we will talk about one important concept of optics. Grade 8 physics gives this phenomenon as the laws of refraction and reflection of light.

Wave and light

Oddly enough it sounds, but light is a wave. “In which seas?” - students will ask. And we will answer: "In electromagnetic." The beginning of this complex system is given by a moving charged object. In the literal sense of the word. If the experimenter electrifies a piece of amber and quickly runs with it, then in the process of movement a very weak and very short electromagnetic field will arise. The source of the large fields that pervade the entire universe are mainly stars. The sun is also an object with a non-zero charge, so the Earth literally "bathes" in the particles and electromagnetic fields created by it. And light is a quantum of the electromagnetic field, which means that the law of reflection can be applied to it.

Reflection, refraction, absorption

So what is the essence of the law? In the following:

1. If a beam of light falls on a smooth surface, then it, normal to the surface at the point of incidence, and reflected light lie in the same plane.
2. The angle of inclination of the incident beam to the normal is equal to the angle of inclination of the reflected light.

Sometimes students are frightened by the incomprehensible word "normal." But it’s not at all terrible. This is just the perpendicular to a given point on the surface. And the normal is most often an imaginary line, it must be thought out in order to solve the problem.

The angle of incidence is equal to the angle of reflection

How harmful is this formulation of the law of reflection of light? Grade 8 often reduces the number of words in school rules in order to better remember them. But even linear optics is an object in which the vector of action and propagation matters. That is, not only the mutual angles of the light beams are important, but also the direction of their propagation. In this case, it is important not to forget that for the incident, reflected image and the normal to the surface at the point of incidence there is only one plane.

Types of reflection

It would seem that this rule cannot be simpler. But here there are some peculiarities:

1. When meeting with a dielectric, light causes dielectric polarization oscillations in its atoms. This leads to the fact that each point of the medium becomes a secondary source of waves. When folded, they generate reflected, refracted, and diffused light.
2. Getting on a conductive material, electromagnetic radiation causes oscillations of electrons. The material tends to compensate for the emerging current, which causes an almost complete reflection. Therefore, the metal is so shiny.
3. Diffuse reflection occurs when a surface has roughness. Their size should exceed the wavelength of the incident radiation. However, a situation may arise in which the shortwave violet radiation is scattered and the longwave red is perfectly reflected.
4. Internal reflection. If light falls from a denser medium to a more rarefied one (for example, from water to air), then at a certain angle the entire beam is reflected back. The law of total reflection is associated with the difference in the refractive indices of light in the medium. His formula is expressed as follows:

• sin j = n ₂ / n ₁

where j is the angle at which full internal reflection occurs, and n ₂ and n ₁ are the refractive indices of two media.

What is reflected and when?

In addition to school lessons and boring tasks, the law of reflection, the formula of which we gave a little higher, can be observed in other cases:

1. When waves of sound bounce off solid surfaces, they come back in the form of an echo. It is because of this effect that children's voices sound louder in a closed courtyard than on the banks of a river. An empty room immediately after repair also gives an echo, and the furniture that is put there later absorbs air vibrations.
2. Reconnaissance ships send ultrasound waves ahead of them, by the speed of reflection of which one can judge the topography of the bottom.
3. Radio waves are reflected from the aircraft, which allows you to determine their location in the air.
4. In a medical study, ultrasound is reflected from the border of the organs and gives specialists the opportunity to judge what is happening inside the person without cutting tissue.

Mirror and China

However, do not think that reflection is the latest invention. As soon as people learned how to get pure metal (bronze), women immediately wanted to know how they look.

To better reflect the material, its surface was polished by hand for a long time. And since it was possible to look only in one direction of the bronze disk, the other was decorated with some kind of pattern.

In ancient China, some craftsmen were able to make mirrors, the mystery of which has not been solved until now. If the sunny bunny from the smooth side of such an object is directed to a white wall or sheet of paper, then in the circle of light ... a picture engraved on the reverse side will appear. Even the modern research methods could not explain the essence of this phenomenon. Assumptions about how this happens are:

1. The pattern is pressed through, then one side is ground, and the difference in the structure of the metal remains.
2. Copper melt is poured into a pre-prepared template, and a thicker layer of metal (where the pattern is convex) solidifies in a slightly different form than a thin element. This difference remains after grinding.
3. The smooth side of the mirror is etched with acid. After processing, the color difference is not noticeable, but in the bright sunlight, the intensity of the reflected image is different.
4. On the mirror part of the object, the drawing is applied with copper of a different grade.
5. The image is cut out on the back of the mirror when the front is already polished to a certain extent. Pressure affects both parts of the subject. The mirror side is covered, as it were, with a series of micro-bulges that correspond to the figure. Another polishing completes the work, giving the created mounds and depressions a smoother look.

It is difficult to believe that in the age of atomic spectroscopy and X-ray research of matter, there are still puzzles related to reflection, but the facts are stubborn things.