Explain critical angle. Light travels in two media A and B with speeds `2 xx 10^(8) ms^(-1)` and `2.4 xx 10^8 ms^(-1)`, respectively. Calculate the critical angle between them.

Explanation

According to Snell.s law,
`("denser")_(mu_("rarer")) =(sin i)/(sin r)`
or `r_(mu_(d)) = (sin i_(c))/(sin 90^(@)) = sin i_( c)`
`rArr r_(mu_(d)) = 1/(sin i_( c))`
As, `v_(d) = 2 xx 10^(8) ms^(-1)`
and `v_(r) = 2.4 xx 10^(8) ms^(-1)`
`therefore sin i_( c) = d_(mu_(r)) =v_(d)/v_(r) = (2 xx 10^(8))/(2.4 xx 10^(8)) = 5/6`
`rArr i_( c) = sin^(-1)(5/6) = 56.4^(@)`