The twinkling of stars is due to atmospheric:

$(a)$ 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.

$(a)$ With the help of a suitable diagram,explain why the Sun is visible to us two minutes before the actual sunrise and two minutes after the actual sunset.
$(b)$ Name the phenomenon responsible for the apparent flattening of the Sun's disc at sunrise and sunset.

Explain why the planets do not twinkle.

Explain why stars twinkle while planets do not.

Due to atmospheric refraction of light,the Sun appears to rise $................$ its actual sunrise.