Question

‘What is diffraction of light’? Explain its two types.

Answer 1

Hint: Shadow forms because of bending of light i.e. light deviates. So on account of diffraction, light deviates from its linear path. Bending generally occurs at the edges of the object. Type of diffraction can be defined based on distance from obstacle and screen. Distance between obstacle and screen leads to change in the wavefront.

Complete step by step answer:
We know that light travels in a straight line and casts the shadow of the object coming in its path shadow when there is a bending of light through an edge. So in the simple language, bending of light is nothing but diffraction of light.

The bending of light near the edges of an obstacle or slit and spreading into the region of the geometrical shadow is called diffraction of light.
Let’s understand this phenomenon by figure.

When a narrow slit AB is placed in the path of light coming from a point source ‘S’, then only part A’B’ of the screen is expected to get illuminated and no light is expected to enter the geometrical shadow.
Similarly when an opaque body AB is placed in the path of light then its district dark geometric shadow A’B’ is projected on the screen.

Fraunhofer and Fresnel are the two types of diffraction.
(I)Fresnel diffraction:
If the source of light and screen of light is kept at a finite distance from obstacle then Fresnel diffraction takes place. In his case, the lens is not used. The light after diffraction appears to converge towards the screen. It can be observed in straight edge, narrow slit etc. the wavefronts considered are spherical and cylindrical.

(II)Fraunhofer diffraction:
If a source of light and screen are effectively kept at infinite distance from obstacle then Fraunhofer diffraction takes place. In this case, lenses are used. Light after diffraction does not appear to converge towards the screen. It can be observed in the single slit, double slits, etc. Wavefronts considered are plane wave fronts.

Note:
Above explanation is for diffraction due to a single slit. Note that diffraction can occur only when the wavelength of light is approximately equal to the width of the slit and when the size of the slit or opaque object is small. All types of waves can go under the phenomena of diffraction. Fraunhofer diffraction is generally used to get a parallel beam of rays. Using a Fresnel lens, you cannot get a parallel beam of rays from the source.