(a) What is meant by the dispersion of white light? Draw a diagram to show dispersion of white light by the glass prism.

(b) Explain why the planets do not twinkle but the star twinkle.

(a) The phenomenon of splitting of white light into its components colors while passing through a prism is known as Dispersion of light. It can be described with the help of following figure:

(b) Twinkling of stars is due to atmospheric refraction of star light. The star light after entering the earth’s atmosphere undergoes refraction in a continuous manner before it reaches the Earth. The atmospheric refraction is due to change in the refractive index at different level in atmosphere. The star light bends towards the normal, the apparent position is different from the actual position of star. Since atmosphere is not stationary and keeps changing. As path of rays of light coming from star varies slightly then the apparent position of star also varies slightly and the amount of light entering the eye flickers. Sometimes, it is brighter and sometimes the star seems fainter. In this way, stars twinkle but planets don’t twinkle because planets are much closer to Earth and are seen as extended sources. If planet is considered as collection of point sources of light, the total amount of light entering the eye on average is zero and therefore the twinkling effect of planets is nullified.