Which of the following is true for the eye defect known as hypermetropia (farsightedness)?

$(a)$ 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.

Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also,indicate the order of the colours of the spectrum obtained.

Name the delicate membrane in the eye having an enormous number of light-sensitive cells.

The twinkling of stars is due to atmospheric:

Which liquid fills the space behind the cornea?