A point source `S` is placed at the bottom of different layers as shown in figure. The refractive index of bottom-most layer is `mu_(0)`. The refractive index of any other upper layer is
`mu(n)=mu_(0)-(mu_(0))/(4pi-18)` where `n=1,2,.....`
A ray of light starts from the source `S` as shown. Total internal reflection takes place at the upper surface of the layer having `n` equal to

Options

A. 3
B. 5
C. 4
D. 6

Answer

C

Explanation

When `n=1` ,
`mu(1)=mu_(0)-(mu_(0))/(4xx1-18)impliesmu(1)gtmu_(0)`
When `n=2`
`mu(2)=mu_(0)-(mu_(0))/(4xx2-18)impliesmu(2)gtmu_(0)`
When `n=4`
`mu_(4)=mu_(0)-(mu_(0))/(4xx4-18)impliesmu(4)gtmu_(0)`
When `n=5`
`mu(5)=mu_(0)-(mu_(0))/(5xx4-18) rArr mu(5) lt mu_(0)`
Clearly , the total internal reflection shall take place a the top of a layer having `n=4`