The phenomenon of interference is inherent in all types of waves: sound, electromagnetic and others. Therefore, if light has wave properties, then the application of two beams of light can lead not only to amplification, but also to weakening of light, or, in other words, interference of light occurs. This means that the combined action of two light beams can lead to the appearance of darkness, or, figuratively speaking, light plus light can give darkness. Experience confirms this conclusion.
It is possible to obtain a system of coherent light waves if the light beam emanating from the source is divided into two beams in some way and then these two beams are brought together, while the light beams travel through different paths; this creates a stroke difference, and when superimposed, the beams interfere.
There are different ways to implement these conditions.
In one of the experiments of the French physicist Fresnel, the light beam of a point source is divided into two beams using two mirrors placed to each other at an angle close to 180 °.
Light rays from the source S go to the screen AA. Direct rays do not hit the screen, as they are delayed by the QC partition.
Light waves come to the screen from the source S, which go along two paths of different lengths and are therefore late relative to each other. The waves that come from S and are reflected by mirrors I and II are two systems of coherent waves SB₁OC₁C₁ and SB₂OC₂C₂, as if coming from the source S₁ and S₂, which are false images of S in mirrors I and II.
In the OS₁₂ space, dark and light stripes alternate.
The described Fresnel experiment on observing such a phenomenon as light interference is fundamentally simple, but it is technically difficult to implement.
The splitting of a light beam into two beams with subsequent superposition on each other also takes place when thin films are illuminated by light rays. Light interference is very easily observed in thin films of soap bubbles. Having received a soap film on a wire frame and illuminating it with a red light from the source, using a collecting lens we project our film onto the screen. On the screen, the film image at first seems evenly lit. But as the film becomes thinner due to water draining (first in the upper and then in other parts of it), alternating horizontal dark and red stripes appear. With further thinning of the film, the observed picture changes: red spots appear in place of dark bands and vice versa. Similar patterns would be observed when illuminating a soap film with any uniform light. The same pattern would be observed when illuminating films of other substances, for example, oil films on the surface of the water.
What phenomena occur on a soap film when it is illuminated with uniform light? Parallel rays of light fall on the film. Reflecting from its upper and lower boundaries and acquiring a stroke difference, interference of light from the rays occurs when superimposed on each other. If you collect them with a lens, then on the screen we will get a series of light stripes, which is divided by dark gaps. When the film is illuminated with white light, the interference pattern is multicolored. This is a consequence of the complexity of white light, which consists of waves of various lengths, which form during the interference maxima and minima of light in various places.
The presence of alternating light and dark bands of monochromatic light, as well as continuous spectra in the case of illumination with white light, indicates its wave properties.
The widest application of light interference has been found in the illumination of optics. What is it?
Light that falls on the lens is partially reflected back; the fraction of reflected light is usually a few percent. Lenses of modern optical technology are lens systems. As a result of reflections on the surface of each lens, a significant attenuation of light occurs. In order to reduce this effect, an interference coating in the form of a thin film is applied to the surface of each lens.
The thickness of the coating is selected so that the reflected waves are shifted by half a wave and, interfering, cancel each other out. Then there will be no reflection loss, and all light energy will pass through the lens. The image will turn out brighter - the optics are “brightened”.