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The length of plane mirror is 1cm. The wavelength of source is 5000 A0 . The number of maxima formed on the large screen are (P is central point on screen)

a
19,999
b
20,000
c
20,001
d
39,998

detailed solution

Correct option is A

An image S' will form at 5mm behind the mirror. Hence, Path difference at P = ∆x = S'P - SP = 10 mm  Also,  ∆x = nλ   [Condition for minima, as mirror introduces phase difference of π] ⇒n= ∆xλ=10×10-35×10-7 = 20,000th minima  At infinity,  ∆x = 0  means minima  Hence, between lower Infinity and point P , 19,999 maxima exists.  No interference occur above point P.

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