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.

(N/A) Ciliary muscles help in changing the focal length of the eye lens.
$(b)$ Cataract: The crystalline lens of the eye is made of proteins arranged in a regular pattern,which makes the lens transparent. When a group of these protein molecules clump together in a region,they make the region opaque. Gradually,this membrane grows,and the whole lens becomes opaque.
$(c)$ The colour of scattered light depends on the size of the scattering particles (air molecules,dust particles,and pollutants) in the atmosphere.
$(d)$ From $25 \ cm$ to infinity.
$(e)$ There is no atmosphere in space,hence there are no air molecules to scatter sunlight,making the sky appear dark.