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For Secondary Minima
( a ) Path difference = nλ ( b ) Linear distance = nDλ / a = nfλ / a ( c ) Angular spread = nλ / a where , n = 1 , 2 , 3 ,.,.
For Secondary Maxima
( a ) Path difference = ( 2n + 1 ) & lamda ; / 2 ( b ) LInear distance = ( 2n + 1 ) D & lamda ; / 2a = ( 2n + 1 ) f & lamda ; / 2a
( c ) Angular spread = ( 2n + 1 ) & lamda ; / 2
Important Points
� A soap bubble or oil film on water appears coloured in white light due to interference of light reflected from upper and lower surfaces of soap bubble or oil film . � In interference fringe pattern all bright and dark fringes are of same width , � In diffraction fringe pattern central bright fringe is brightest and widest . and I remaining secondary maximas are of gradually decreasing intensities .
� The difference between interference and diffraction is that the interference is the superposition between the wavelets coming from two coherent sources while the diffraction is the superposition between the wavelets coming from the single wavefront
Polarisation
The phenomena of restructuring of electric vectors of light into a single direction is called polarisation .
Ordinary light has electric vectors in all possible directions in a plane perpendicular to the direction of propagation of light .
When ordinary light is passed through a tourmaline , calcite or quartz crystal the transmitted light have electric vectors in a particular direction parallel to the axis of crystal . This light is plane polarised light .
[ A plane containing the vibrations of polarised light is called plane of vibration . A plane perpendicular to the plane of vibration is called plane of polarisation .] Polarisation can take place only in transverse waves .