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Fresnel’ s Biprism
It is a combination of two prisms of very small refracting angles placed base to base. It is used to obtain two coherent sources from a single light source.
Lyod’ s Mirror
The shape of interference fringes are usually hyperbolic. When screen is held at 900 to the line joining foci of the hyperbola, the fringes are circular.
When distance of screen( D) is very large compare to the distance between the slits( d), the Cringes are straight.
Diffraction
The bending of light waves around the corners of an obstacle or aperture is called diffraction of light.
The phenomenon of diffraction is divided mainly in the following two classes
( a) Fresnel class( b) Fraunhofer class
S. No Fresnel Class 1 The sourceis at a finite distance
2 No optJcals are required. 3 Fringes are not sharp and well defined.
Fraunhofer Class The source is at infinite distance
Opticals are in the form of Collimating lens and focusing lens are required.
Fringes are sharpand well defined.
Fraunhofer Diffraction at a Single Slit
Linear Width 0f central maximum 2Dλ / a = 2fλ / a Angular width of central maximum = 2λ / a
where, λ = wavelength of light, a = width of single slit, D = distance of screen from the slit and f = focal length of convex lens.