Twinkling of stars is due to atmospheric _________. Explain briefly.

(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds [2 Marks]

Option (b) is correct. [1 Mark]
Stars twinkle because of variation in the density of earth's atmosphere. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence twinkle.
Stars (except for the sun) appear as tiny dots in the sky as they are many light years away from earth. As their light travels through the many layers of the earth's atmosphere, the light of the star is bent (refracted) many times and in random directions. This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling).
Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so refracts more. [1 Mark]