What is meant by the spectrum of white light?

What is the time difference between the actual sunset and the apparent sunset?

$(a)$ 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.

Name the part of the eye where the image is formed by the eye lens. What is the nature of the image formed? How is this image sent to the brain?

What is meant by the near point of the eye?

Which lens is used to correct the defect of myopia (nearsightedness)?