# A slit S is illuminated by a monochromatic suorce of light to give two coherent sources P1 and P2 .

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A slit S is illuminated by a monochromatic suorce of light to give two coherent sources P1 and P2 . These give a dark band at the point R on the screen as shown in figure.

1. Write the formula to find out bandwidth.

2. What relationship must exist between the length P1 R and P2 R?

3. Can interference fringes be produced by using two identical bulbs?

4. If the distance between P1 and P2 is 1 mm and the screen is placed in 1m away, what is the fringe separation for a light of wavelength 5 × 10-7 m?

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1. β = $\frac{λD}{d}$

2. The condition for dark fringe is P2R – P1R = (2n -1)$\frac{λ}{2}$ where n = 1,2, 3, etc.

3. No. They are not coherent sources.

4.  β =  $\frac{λD}{d}$ = $\frac{1\times5\times10^{-7}}{10^{-3}}$ = 5 x 10-4 = 0.5 mm