In Youngs 's double slit experiment, the fringe width obtained by using monochromatic light of wavelength 6000 Å is 2 mm. if the whole apparatus is immersed in water of R.I. 1.33, then the change in fringe width will be

Options

A. 1 mm
B. 1.2 mm
C. 1.5 mm
D. 0.5 mm

Answer

D

Explanation

`lamda=(Xd)/(D)" "therefore X=(lamdaD)/(d)" "therefore X prop lamda`
When the whole apparatus is dipped in water, X is decreased.
`R.I.(n)=(lamda_(air))/(lamda_(water))" "therefore lamda_(w)=(lamda_(a))/(n)=(lamda_(a))/(4//3)=(3)/(4) lamda_(a)`
`because Xprop lamda," "therefore X_(water)=(3)/(4)(X_(air))=(3)/(4)xx2=1.5mm`
`therefore` The fringe width will decrease by (2-1.5)=0.5mm.