Which two colors of light combine to form the color maroon?

What is the range of vision for a normal human eye?

$A$ bifocal lens consists of both concave and convex lenses. Bifocal lenses are required by a person suffering from $.............$.
$P$ - Myopia (nearsightedness)
$Q$ - Hypermetropia (farsightedness)
$R$ - Presbyopia

Name the condition resulting due to the eye lens becoming cloudy.

$A$ prism causes dispersion of white light while a rectangular glass block does not. Explain.

Why does the sky appear dark to passengers flying at high altitudes?