A source of light of wavelength 5000Å is placed as shown in figure.
Considering interference of direct and reflected rays, determine the position of the
region where the fringes will be visible and calculate the number of fringes.
` .

Answer

D

Explanation

Interference will be obtained between
direct rays from S and reflected rays from S' (image
of S on mirror).
Since, the reflected rays will lie between region P and
Q on the screen. So, interference is obtained in this
region only. From geometry will can show that,
OP = 1.9cm and OQ = 3.9cm.
`:. PQ=2cm`
`Fringe width, omega=(lambdaD)/d = ((5000 xx 10^(-8))(200))/(2 xx 10^(-1)) cm = 0.05 cm`
Total number of fringes in the region PQ,
`N=(PQ)/omega = 2/0.05 = 40`