The light diffraction is the name given to the distortion of a light beam when it hits a small object or a small opening in a screen. It was the Italian Francesco Maria Grimaldi who gave the name of diffraction to this phenomenon and the first to study it in 1665.
When the object or slot that intercepts the light beam is on the order of tenths of a millimeter or less, the cast shadow is not accurate. Rather, it diffuses around what should be its geometric shadow. This is because the light beam is deflected and spreads around the edges of the obstacle..
The figure above shows a very particular pattern of alternating light and dark areas. Produced by light from a laser pointer (wavelength 650nm) passing through a 0.1mm x 0.1mm square slot and projected onto a screen.
This pattern formation phenomenon is also observed in sound waves and waves on the surface of the water, as well as in radio waves and X-rays. That is why we know that it is an eminently wave phenomenon..
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In a monochromatic beam of light (containing a single wavelength) such as laser light, the diffraction of the incident light beam on the obstacle forms a pattern of light and dark bands when projected onto a screen.
This arrangement of light and dark areas is called diffraction pattern.
Diffraction is explained in a classical way, according to Fresnel-Huygens principle.
It comes from the superposition of the spherical waves emanating from the edge of the obstacle and from the other points of the wave front that adjoins the edges, in such a way that an interference occurs between the waves from this set of secondary sources.
When two or more waves coincide in the same place in space, interference occurs between them. It can then happen that their respective amplitudes are added or subtracted, after which each one goes its own way..
It all depends on whether the waves coincide in phase. If so, the amplitudes add up, while in those places where the waves are in phase or counter phase, the amplitude decreases or cancels out..
That is why the diffraction pattern has light and dark areas..
Unlike the phenomenon of light interference, in which the number of wave sources is two or three, in the case of diffraction the number of secondary sources of spherical waves is very large and tends to form a continuum of sources..
Wave interference in diffraction is more noticeable if the source has a single wavelength and all the photons that make up the light beam are in phase, as is the case with light from a laser..
The speckle interferometry is one of the practical applications of the phenomenon of light diffraction.
When a surface is illuminated with laser light, the wavefronts of the light reflected from the surface are in phase, but they shift out of phase after traveling their way to the plate or screen on which the image is recorded..
There a speckled diffraction pattern is produced (speckle in English), which gives information on the surface from which the reflected photons come.
In this way, flaws or fractures can be detected in a part, which would hardly be visible to the naked eye.
Knowledge of the diffraction patterns present in photographic or digital images of astronomical objects: stars or asteroids, serves to improve the resolution of astronomical images.
The technique consists of collecting a large number of images of the same object that individually are of low definition or brightness..
Then, when processed computationally and extracting the noise from diffraction, they result in a higher resolution image.
This is how it is possible to show details that were previously masked in the originals, precisely due to light diffraction..
Diffraction is a phenomenon that almost all of us surely observe, but we do not always properly identify its origin. Here are some examples:
The rainbow is mainly caused by the superposition of the refracted and reflected waves inside the fine water droplets.
They make up a very large set of secondary light sources, whose waves interfere, forming the colorful rainbow pattern that we admire so much after the rain..
Light bouncing off a CD or DVD also forms striking colorful patterns. They have their origin in the phenomenon of the diffraction of the light reflected by the sub-millimeter grooves that make up the tracks..
The hologram that often appears on credit cards and branded products forms a three-dimensional image.
It is due to the superposition of the waves coming from the innumerable printed reflective points. These points are not randomly distributed, but were formed by the diffraction pattern of the original object, which was illuminated with laser light and later engraved on a photographic plate..
Sometimes halos or rings can be seen around the Sun or the Moon.
They are formed thanks to the fact that the light coming from these celestial bodies bounces or is reflected in an innumerable amount of particles or crystals formed in the upper atmosphere..
They in turn act as secondary sources and their superposition gives rise to the diffraction pattern that forms the celestial halo..
The iridescence of some surfaces such as soap bubbles, or the translucent wings of some insects, is explained by light diffraction. On these surfaces the tones and colors of light observed vary depending on the angle of observation..
The photons reflected in the thin semi-transparent layers constitute a large set of light sources that interfere constructively or destructively.
Thus, they form the patterns corresponding to the different wavelengths or colors, of which the light from the original source is composed..
Thus, only the wavelengths coming from certain trajectories are observed: those that go from the reflected points, to the observer's eye and that have an integer difference in wavelengths..
Wavelengths that do not meet this requirement are canceled and cannot be observed.
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