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

Options

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

Answer

C

Explanation

The refractive index of a medium increases with frequency .e.g. In the visible spectrum `mu_v gt mu_R` because `v_v gt v_R` .
From Snell's law , `mu=(sin i)/(sin r)` and for critical angle `mu=1/(sin C)`
Hence , for same I, for high frequency waves (high `mu`), sin r 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 10 MHz.