Four waves A, B, C, D of frequencies 6 MHz. 8 MHz, 10 MHz and 4 MHz respectively are beamed in the same direction, communicate via sky waves. Which one of these is likely travel the longest distance in the ionosphere before suffering total internal reflection ?

Options

A. A
B. B
C. C
D. D

Answer

C

Explanation

Refractive index of as medium increases with frequency e.g. for the visible spectrum `mu_(v)gtmu_(R)` because `v_(v)gtv_(R)`
Applying Snell's law we get
`mu=(sini)/(sinr)`
and critical angel `c=sin^(-1)(1/(mu))`
hence for same `i` and for high frequency waves (high `mu`), `sinr` and hence `r` is decreased i.e. bending of the waves is decreased. Hence the condition of total internal reflection is attained after travelling the longest distance in the ionosphere by the wave C having the highest frequency of 10MHz.