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Diffraction of Light

Bending of light rays from sharp edges of an opaque obstacle or aperture and its spreading in the geometricle shadow region is defined as diffraction of light or deviation of light from its rectilinear propogation tendency is defined as diffraction of light.

Diffraction was discovered by Grimaldi & it was theoretically explained by Fresnel.

Diffraction from an obstacle and aperture

Diffraction is possible in all type of waves like Mechanical and EM waves.

Diffraction depends on two factors :
(i) Size of obstacles or aperture (ii) Wave length of the wave

Diffraction - The Properties of Waves

Condition of diffraction

Size of obstacle or aperture should be nearly equal to the wave length of light.

If size of obstacle is much greater than wave length of light, then rectilinear motion of light is observed.

It is practically observed when size of aperture or obstacle is greater than 50λ then obstacle or aperture does not shows diffraction.

Sound wave shows more diffraction as compare to light rays because wavelength of sound is high (16 mm to 16m). So it is generally diffracted by the objects in our daily life.

Diffraction of ultrasonic wave is also not observed as easily as sound wave because their wavelength is of the order of about 1 cm.

Diffraction of radio waves is very conveniently observed because of its very large wavelength
(2.5 m to 250 m). X-ray can be diffracted easily by crystal. It was discovered by Lave.

Diffraction of sound from window

(i) There are two type of diffraction of light : (a) Fresnel’s diffraction. (b) Fraunhofer’s diffraction.

( a ) Fresnel diffraction
If either source or screen or both are at finite distance from the diffracting device (obstacle or aperture), the diffraction is called Fresnel diffraction.

( b ) Fraunhofer diffraction
Fraunhofer diffraction is a particular limiting case of Fresnel diffraction. In this case, both source and screen are effectively at infinite distance from the diffracting device.

Example :- Diffraction at single slit


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