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What is interference and diffraction of light

The wave nature of light doesn't produce any noticeable effects in ordinary lighting, but two phenomena have applications in certain technologies.

The first is called interference. This phenomenon occurs when two separate yet "coherent" light sources illuminate the same screen. "Coherence" means that both sources radiate light of exactly the same wavelength and with a fixed phase relationship. When the light from these two sources is combined, the result is that in some areas of the screen, the two light waves are in phase and add to each other, while in other areas, they are out of phase and cancel each other. The interference between the two sets of light waves typically appears as alternating light and dark stripes on the screen. Coherent light sources in practice are produced by splitting a single light beam, typically using glass coated with a partially reflective film. Modern lighting technology uses interference in dielectric filters, which selectively reflect or transmit portions of the spectrum. These filters also exploit the effect that when a light beam is perpendicularly incident on a surface with a higher refractive index, the phase angle of the reflected light changes by 180°.

A second phenomenon caused by the wave nature of light is called diffraction, which occurs when light bends around the edges of obstacles. While the diffraction effect is generally subtle and imperceptible to the naked eye, it is crucial in optical instruments such as high-magnification microscopes and telescopes. For example, diffraction gratings used in instruments for examining light spectrums utilize both diffraction and interference effects.

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