# What is interference and diffraction?

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Looking at the glowing holographic image, most of us hardly remember physical terms. “diffraction” and “Interference of light waves”

But thanks to the study of these concepts, it became possible to create holograms.

## What is Light Diffraction?

Word “diffraction” derived from Latin Diffractuswhich means literally “Swinging around an obstacle”… As you know, light has a wave nature, and its rays obey wave laws. Diffraction in physics is called optical phenomena that occur when light waves propagate in an optically inhomogeneous medium with opaque inclusions.

The wave nature of light determines its behavior when bending around obstacles. If the obstacle is many times longer than the light wavelength, the light does not bend around it, forming a shadow zone. But in cases where the dimensions of the obstacles are commensurate with the wavelength, the phenomenon of diffraction occurs. In principle, any deviation from geometric optical laws can be attributed to diffraction.

## Wave interference

If we place an opaque screen in front of the light source and make a dotted hole in it, then the rays of light penetrating through this point on the next screen, located parallel to the first one, will be displayed in the form of concentric rings with alternating light and dark circles. This phenomenon in physics is called Fresnel diffraction, after the name of the scientist who first discovered and described it.

By changing the shape of the hole and making it slit, we get a different picture on the second screen. The light beams will be arranged in a series of light and dark stripes, like on a store barcode. The diffraction of light by a slit-like hole was first described by the German physicist Fraunhofer, after whom it is still called.

Scientists were able to explain the decomposition of a light wave into light and dark areas using the concept of interference. Several sources of wave oscillations, if their oscillation frequencies are coherent (the same or multiple of each other), can enhance each other’s radiation, but can also weaken, depending on the coincidence of the oscillation phases. When bending around obstacles and the appearance of secondary waves, their interference comes into play. In areas where the phases of the waves coincide, there is an increased illumination (bright light stripes or circles), and where they do not coincide, the illumination is reduced (dark areas).

## Diffraction grating

If we take a transparent plate and put on it a series of parallel opaque lines at the same distance from each other, then we get a diffraction grating. When a plane light front passes through it, diffraction is formed on opaque strokes. The secondary waves, mutually attenuating and amplifying, form diffraction minima and maxima, which can be easily detected on a screen placed behind the grating.

In this case, not only the deflection of light rays occurs, but also the decomposition of white light into color spectral components. In nature, the color of the wings of butterflies, plumage of birds, and snake scales necessary for camouflage is often formed due to the use of diffraction and interference optical phenomena, and not due to pigments.

## Holograms

The hologram principle was invented in 1947 by the physicist D. Gabor, who later received the Nobel Prize for his invention. Three-dimensional, i.e. 3D image of an object can be captured and recorded, and then reproduced using laser beams. One of the light waves is called the reference wave and is emitted by the source, and the second is the object wave and is reflected from the recorded object.

On a photographic plate or other material intended for recording, a combination of light and dark stripes and spots is fixed, which reflect the interference of electromagnetic waves in this area of ​​space. If light with a wavelength corresponding to the characteristics of the reference wave is directed onto the photographic plate, then it is converted into a light wave, which is close in characteristics to the object wave. Thus, a three-dimensional image of a fixed object is obtained in the luminous flux.

Today, still holograms can be recorded and played back even at home. This requires a laser beam, a photographic plate and a frame that reliably holds these devices, as well as the recording object, in immobility. For a home hologram, a laser pointer beam with the focusing lens removed is perfect.