The time required for the light to pass through a glass slab ( refractive index=1.5) of thickness `4 mm` is (c=`3xx10^(8)ms^(-1)` speed of light in free space)

Options

A. `10^(-11)s`
B. `2xx10^(-11)s`
C. `2xx10^(11)s`
D. `2xx10^(-5)s`

Answer

B

Explanation

(b) we have `n_(a)c_(a)=n_(g)c_(g)`
`n_(g)/n_(a)=c_a/c_g`
`3/2=(3xx10^(8))/c_(g)`
`c_(g)=2xx10^(8)`
Time = `"Distance"/"Speed"`
`t=(4xx10^(-3))/(2xx10^(8))`
`t=2xx10^(-11)s`