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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 .