This interference film is used to measure the thickness of slides, paper, etc. The arrangement is as shown in fig. For the sake of clarity, the two strips are shown thick. Consider the wedge formed in between strips 1 and 2. If the interference pattern because of the two waves reflected from wedge surface is observed, then from the observed data we can compute thickness of paper, refractive index of the medium filled in wedge, number of bonds formed, etc.
Considre the strips to be thick as compared to wavelength of ligth and light is incident normally.
Neglect the effect due to reflection from top surface of strip 1 and bottom surface of strip 2. Take `L = 5 cm` and `lambda_(air) = 40 nm`.

For data in question53 determine the distance of point B from the 20th dark band. Counting of dark point has to start from the contact point.

Options

A. 4 cm
B. 3 cm
C. 5 cm
D. 2cm

Answer

a

Explanation

For 20th dark band,
`x = 20 xx 5 xx 10^(-4) m = 1 m`
So, required separation `= S - 1 = 4 cm`