Today we will talk about the law of reflection of light. We will also highlight the part of linear optics to which this phenomenon applies.
School and light
Children go to the first grade impatiently. They are interested in what it means to study, they are captured by fuss with textbooks and notebooks. But discipline is a strict thing. Yes, and the psychological laws of a closed group of children are quite cruel. Therefore, older students associate with school only a reluctance to go there. However, with a creative approach to the knowledge itself, you can change the way you look at the world of lessons and diaries. Today we will talk about one important concept of optics. Physics grade 8 gives this phenomenon as the laws of refraction and reflection of light.
Wave and light
As strange as it sounds, light is a wave. "What seas?" the students will ask. And we will answer: “In electromagnetic”. This complex system begins with 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 large fields that permeate the entire universe are inmostly stars. The Sun is also an object with a non-zero charge, so the Earth literally “baths” 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:
- If a beam of light falls on a smooth surface, then it, the normal to the surface at the point of incidence and the reflected light lie in the same plane.
- The angle of inclination of the incident beam to the normal is equal to the angle of inclination of the reflected light.
Sometimes schoolchildren are frightened by the incomprehensible word "normal". But it's not terrible at all. It is just a 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? 8th grade often reduces the number of words in school rules in order to better remember them. But even linear optics is a subject 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, there is only one plane at the point of incidence.
Types of reflection
It would seem that this rule cannot be simpler. But here there are some peculiarities:
- Meeting with a dielectric, light causes oscillations in its atomsdielectric polarization. This leads to the fact that each point of the medium becomes a secondary source of waves. When combined, they generate reflected, refracted and diffused light.
- When electromagnetic radiation hits a conductive material, it causes electrons to oscillate. The material tends to compensate for the resulting current, which results in an almost total reflection. That's why the metal is so shiny.
- Diffuse reflection occurs when a surface has roughness. Their size must exceed the wavelength of the incident radiation. However, a situation may arise in which short-wavelength violet radiation is scattered, while long-wavelength red radiation is perfectly reflected.
- Internal reflection. If light falls from a denser medium into 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 related to the difference in the refractive indices of light in a medium. Its formula is expressed as follows:
- sin j=n2 / n1
where j is the angle at which total internal reflection occurs, and n2 and n1 are the refractive indices of the two media.
What and when is reflected?
In addition to school lessons and boring tasks, the law of reflection, the formula of which we have given a little higher, can be observed in other cases:
- When sound waves bounce off solid surfaces, they bounce back as an echo. It is because of this effect that children's voices sound louder in an enclosed yard than outside.riverbank. An empty room immediately after repair also echoes, and the furniture that is put there later absorbs air vibrations.
- Reconnaissance ships launch ultrasonic waves in front of them, the speed of reflection of which can be used to judge the bottom topography.
- Radio waves are reflected from aircraft, which allows you to determine their location in the air.
- In a medical examination, ultrasound is reflected from the border of organs and gives specialists the opportunity to judge the processes occurring inside a 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 what they looked like.
To make the material reflect better, its surface was polished by hand for a long time. And since it was possible to look only in one direction of the bronze disc, the other was decorated with some pattern.
In ancient China, some masters were able to make mirrors, the mystery of which has not been solved until now. If a sunbeam from the smooth side of such an object is directed to a white wall or a sheet of paper, then in the circle of light … the picture engraved on the reverse side will appear. The essence of this phenomenon could not be explained even by modern research methods. Guessing how this happens is:
- The pattern is pressed through, then one side is ground, and the difference in the structure of the metal remains.
- Copper melt is poured into a template prepared in advance, anda thicker layer of metal (where the pattern has a bulge) solidifies in a slightly different shape than a thin element. This difference remains even after polishing.
- The smooth side of the mirror is etched with acid. After processing, the difference in color is not noticeable, but the intensity of the reflected image is different in bright sunlight.
- The pattern is applied to the mirror part of the object with a different grade of copper.
- The image is cut out on the back of the mirror when the front has already been sanded to a certain extent. Pressure acts on both parts of the object. The mirror side is covered, as it were, with a series of micro-bulges that correspond to the pattern. Another sanding finishes the job, giving the created bumps and valleys a smoother look.
It's hard to believe that in the age of atomic spectroscopy and X-ray research of matter, there are still mysteries related to reflection, but facts are stubborn things.
