Question

Two waves A and B of frequencies 2MHz and 3 MHz, respectively are beamed in the same direction for communication via sky wave. Which one of these is likely to travel longer distance in the ionosphere before suffering total internal reflection?

Answer 1

Angle of Incidence>Critical Angle

Explanation:

We know that refractive index p of a layer is  

$\mu = \mu_0 \sqrt {1 - \frac{81.45}{v^2}}$

• The refractive index of wave B is more than refractive index of wave A because frequency of wave B is more than wave A (as refractive index increases with frequency increases).

$\frac {Sin\ i}{sin\ r} = \mu$ (lesser the value of r larger the value of µ )

• For higher frequency wave (i.e., higher refractive index) the angle of refraction is less, i.e., bending is less. So, wave B travels longer distance in the ionosphere before suffering total internal reflection.

Importance point: Refractive index of a medium is that characteristic which decides speed of light in it.

Dependence of Refractive index:

(i) Nature of the media of incidence and refraction.

(ii) Colour of light or wavelength of light.

(iii) Temperature of the media: Refractive index decreases with the increase in temperature.

• Total internal reflection: When a ray of light goes from denser to rarer medium it bends away from the normal and as the angle of incidence in denser medium increases, the angle of refraction in rarer medium also increases and at a certain angle, angle of refraction becomes 90°. This angle of incidence is called critical angle (C).

• When angle of incidence exceeds the critical angle then light ray comes back into the same medium after reflection from interface. Total Internal Reflection is the term given to this Process.