Due to atmospheric refraction, the sun is visible about $2$ minutes before the actual sunrise and about $2$ minutes after the actual sunset. By how much time is the day lengthened due to this phenomenon?

Which optical phenomenon is responsible for the twinkling of stars?

$(a)$ If a person wears a lens of power $-6\, D$ for distant vision and for correcting his near vision he needs a lens of $+2\, D$. Determine the focal length of the lenses in both the cases.
$(b)$ Give reasons for the following natural phenomena:
$(i)$ Stars twinkle.
$(ii)$ Planets do not twinkle.
$(iii)$ Stars appear raised in the sky.

What is the twinkling of stars due to?

How does atmospheric refraction take place? Why do stars twinkle,but planets do not?

Why do stars twinkle?