Interference of light

The phenomenon of interference is inherent in all types of waves: sound, electromagnetic and other. Therefore, if the light has wave properties, then the superposition of two beams of light can lead not only to amplification, but also to the attenuation of light, or, in other words, interference of light occurs. So, the joint 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 a light beam emanating from a source is divided into two beams by any means and then both of these beams are brought together, with the light beams passing different paths; This creates a path difference, and when applied, the beams interfere.

There are different ways to implement these conditions.

In one of the experiments of the French physicist Fresnel, the beam of light from a point source is divided into two beams by means of two mirrors placed at an angle close to 180 ° to each other.

Light rays from the source S go to the screen AA. Direct rays do not get to the screen, since they are trapped by the CC partition.

To the screen from the source S light waves come that go along two paths of different lengths and therefore are late relative to each other. The waves that go from S and are reflected by mirrors I and II represent two systems of coherent waves SB₁OC₁C₁ and SB₂OC₂C₂, as if emanating from the source S₁ and S₂, which are false images of S in mirrors I and II.

In the space of OS₁C₂, dark and light stripes alternate.

The described Fresnel experiment on observing such a phenomenon as interference of light is basically simple, but it is difficult to technically implement it.

The splitting of the light beam into two beams, followed by superposition on each other, also takes place when the light rays are illuminated by thin films. Interference of light in thin films of soap bubbles is very easy to observe . Having received a soap film on the wire frame and lighting it with red light from the source, with the help of a collecting lens, we project our film onto the screen. On the screen, the film image at first appears uniformly lit. But as the thinning of the film due to the flow of water (first in the upper and then in other parts of it), alternating horizontal dark and red stripes appear. With a further refinement of the film, the observed picture changes: in the place of dark bands, the red ones appear and vice versa. Similar pictures would be observed when lighting a soap film with any uniform light. The same pictures would be observed when the films of other substances are illuminated, for example, oil films on the water surface.

What phenomena occur on a soap film when it is illuminated with a homogeneous light? Parallel rays of light fall on the film. Reflecting from its upper and lower boundaries and gaining a path difference, interference of the light of the rays occurs when superimposed on each other. If they are assembled by a lens, then on the screen we get a series of bright bands, which is separated by dark gaps. When the film is covered with white light, the interference pattern turns out to be multicolored. This is a consequence of the complexity of white light, which includes waves of different lengths, which form interference and light maxima in various places.

The presence of alternating light and dark bands of monochromatic light, as well as continuous spectra in the case of white light, indicates its wave properties.

The widest application of light interference was found in the enlightenment of optics. What is it?

The light that falls on the lens is partially reflected back; The fraction of reflected light is usually a few percent. The objectives 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 chosen so that the reflected waves are shifted to half-waves and, interfering, extinguish each other. Then there will be no loss of reflection, and all the light energy will pass through the lens. The image will be brighter - the optics are "enlightened".