The lens of the eye can become thicker and thinner. Why is this flexibility useful?

A prism $ABC$ (with $BC$ as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?

Light is incident normally on face $AB$ of a prism as shown in figure. A liquid of refractive index $p$ is placed on face $AC$ of the prism. The prism is made of glass of refractive index $\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$. The limits of $\mu$ for which total internal reflection takes place on face $AC$ is:

Consider the given statements and select the correct option which correctly identifies true (T) and false (F).
$\left(\mathrm{i}\right)$ The focal length of the eye lens increases when eye muscles are relaxed and lens becomes thinner.
$\left(\mathrm{ii}\right)$ Stars appear to twinkle because of movement of air only.
$\left(\mathrm{iii}\right)$ In the dim light, the size of pupil increases.
$\left(\mathrm{iv}\right)$ The blind spot on retina has high concentration of nerve ending.