To correct astigmatism,we use a cylindrical lens.

Which of the following two colors are complementary colors?

How is the eye lens held in its position?

$(a)$ Explain the phenomenon of scattering of light. State the factor on which the colour of scattered light depends.
$(b)$ List any two natural phenomena based on the scattering of light.

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.