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`.

In question 53, if air wedge has been filled with a medium having refractive index 1.3 then find the number of bright bands.

Options

A. 199
B. 99
C. 499
D. 130

Answer

d

Explanation

Here `lambda_("air")` has to replaced by `lambda_("medium")`
`lambda_("medium") = (lambda_("air"))/(mu) = (400)/(1.3) nm`
For bright band, `2 t =(2n - 1) (lambda_("meduim"))/(2)`
`(2 h x)/(L) = (2n - 1) (lambda_("meduim"))/(2)`
`x = (2n - 1) [(400 xx 10^(-9) xx 5 xx 10^(-2))/(2 xx 1.3 xx 2 xx 20 xx10^(-6))]`
For maximum `n, x = L`
`n_(max)`
`= 130.5`