In Young's double-slit experiment, 30 fringes are obtained in the field of view of the observing telescope, when the wavelength of light used is `4000 Å`. If we use monochromatic light of wavelength `6000 Å`, the number of fringes obtained in the same field of view is

Options

A. 30
B. 45
C. 20
D. none of these

Answer

c

Explanation

`30 beta = n beta'`
`implies 30(D xx 4000)/(d) = n(D xx 6000)/(d) implies n = 20`