ICSE Class 10 Answered

Why does stars appear to twinkle in the sky due to change in the refractive index of the air with temperature explain with the help of a diagram

Asked by Kanwaranita10 | 04 Aug, 2019, 07:41: AM

Expert Answer

The twinkling of a star is due to atmospheric refraction of starlight. As the light ray enters from sky to earth,

there is a tmperature gradient in the path. Far away from earth, temperature is cold. Temperature is hot near the earth .

The hotter atmosphere is less dense than the cooler atmosphere and has a refractive index slightly less than

that of the cooler atmosphere. Since the physical conditions of the refracting medium (atmosphere) are not
stationary, the apparent position of the object, as seen through the hotter region of atmosphere, fluctuates.

This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) on a small scale in our
local environment.

The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.

The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends

starlight towards the normal, the apparent position of the star is slightly different from its actual position.

The star appears slightly higher (above) than its actual position when viewed near the horizon (see figure).

Further, this apparent position of the star is not stationary, but keeps on changing slightly,

since the physical conditions of the earth’s atmosphere are not stationary as explained above.

Since the stars are very distant, they approximate point-sized sources of light. As the path of rays
of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the

amount of starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter,

which is the twinkling effect.

Answered by Thiyagarajan K | 04 Aug, 2019, 11:37: AM