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Bright bands positions

D     =         --------(3)

but                    D    =   ml ;    m = 0, 1, 2, 3,------        
              For Constructive Interference

but              =  ml

Thus, Bright bands occur at


distances from the Central bright band at 'O'

Dark bands positions

         D     =                        ------- (3)

but D = (2K + 1) l/2 , K = 0, 1, 2, 3, ..... For Destructive Interference

Thus, Dark bands occur at


from the central bright band t 'O'

Band width : X

From (4) or (5) it is obvious that

   xm+1  -  xm    =    xk+1  -   xk    =    lD   =    X

where X = Distance between any consecutive or Dark bands,
                  is called band width.

Equation (6) can be re-written as

By using Young's double slit or Fresnel's Bi-prism or Lloyd's single mirror method, the band width X and separation d can be measured with the help of an optical bench, for a given distance D and hence the wave length l can be determined.


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18.1 Interference of light
18.2 Diffraction of Light
18.3 Polarization of light

Chapter 19

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