Question

A beam of light consisting of two wavelengths 6500Å and 5200Å is used to obtain interference fringes in YDSE. The distance between slits is 2mm and the distance of the screen form slits is 120 cm. What is the least distance from central maximum where the bright due to both wavelengths coincide?
A. 0.156cm
B. 0.312cm
C. 0.078cm
D. 0.468cm

Answer 1

Correct Answer - A
`(n_1lambda_1D)/d = (n_2lambda_2D)/d`
or `n_1/n_2 = lambda_1/lambda_2 = 5200/6500= 4/5 `
` :.` 4th maxima of `lambda_1` coincides with 5th maxima
of `lambda_2`.
` y_min = (4lambda_1 D)/d `
` = (4 xx 6500 xx 10^-10 xx 1.2)/(2 xx (10^(-3)))`
` = 1.56 xx 10^(-3)m = 0.156 cm ` .