$(a)$ How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
$(b)$ State any two effects of atmospheric refraction on the sun as observed from the Earth.

(A-D) Atmospheric refraction is the change in the direction of propagation of light rays as they travel through the atmosphere due to variations in the density of different air layers. The Earth's atmosphere is not uniform; it is optically and molecularly denser near the surface and becomes rarer at higher altitudes,causing the velocity of light to change from layer to layer. Factors such as varying concentrations of gas molecules,dust particles,and temperature differences (hot air is optically rarer than cold air) create layers of different optical densities,leading to refraction.
Stars appear to twinkle because light from distant stars passes through these varying layers of the atmosphere. Fluctuations in the atmosphere cause the amount of starlight reaching the eye to vary,making the star appear alternately bright and dim,which results in the twinkling effect.
Planets are much closer to the Earth and appear as extended sources rather than point sources. The high intensity of light from planets means that slight atmospheric refraction does not cause significant changes in their perceived brightness,so they do not twinkle.
$(b)$ Two effects of atmospheric refraction on the Sun as observed from the Earth are:
$1$. Advance sunrise: The Sun is visible about $2$ minutes before the actual sunrise.
$2$. Delayed sunset: The Sun remains visible for about $2$ minutes after the actual sunset.