Give one cause of long-sightedness (hypermetropia).

Which part of the eye controls the amount of light entering the eye?

$A$ man wearing glasses of focal length $+1\, m$ cannot see objects beyond $1\, m$. What is the defect in his eye?

Why is the blind spot so called?

Account for the following:
$(a)$ Part of the human eye that helps in changing the focal length of the eye lens.
$(b)$ The condition resulting due to the eye lens becoming cloudy.
$(c)$ The factors on which the colour of the scattered white light depends.
$(d)$ The range of vision of a normal eye.
$(e)$ The sky appears dark to the astronauts in space.

$A$ person cannot see distinctly objects kept beyond $2\, m$. This defect can be corrected by using a lens of power ........ $D$.