In a single slit diffraction, the width of slit is `0.5 cm`, focal lengths of lens is 40 cm and wavelength of light is `4890Å`. Distance of first dark fringe is

Options

A. `2 xx 10^(-5) m `
B. `4 xx 10^(-5) m`
C. `6 xx 10^(-5) m`
D. `8 xx 10^(-5) m`

Answer

B

Explanation

`a sin theta = n lambda "a" . (Y)/(D) = nlambda , ` But = d = f
`"a". (Y)/(D) = n lambda Y = (n lambda f)/(a)`