What is the least distance of distinct vision of a normal human eye (in $cm$)?

Define dispersion of white light and name the colours of white light in order.

An old person is unable to see clearly nearby objects as well as distant objects. $(i)$ What defect of vision is he suffering from? $(ii)$ What kind of lens will be required to see clearly the nearby as well as distant objects? Give reasons.

Give one cause of long-sightedness (hypermetropia).

Due to atmospheric refraction, the sun is visible about $2$ minutes before the actual sunrise and about $2$ minutes after the actual sunset. By how much time is the day lengthened due to this phenomenon?

Give one cause of near-sightedness (myopia).