The slits in a double-slit interference experiment are illuminated by orange light `(lambda = 60 nm)`. A thin transparent plastic of thickness t is placed in front of one of the slits. The nunber of fringes shifting on screen is plotted versus the refractive index `mu` of the plastic in graph shown in figure. The value of t is

Options

A. 4.8 mm
B. `640 mu m`
C. `24 mu m`
D. none of these

Answer

c

Explanation

N = no. of fringes shifted
shift of central maxima `= n lambda = (mu - l) t`
`implies t = 24 mu m`.