The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to

Explain why the planets do not twinkle.

The least distance of distinct vision for a young adult with normal vision is about ...... $cm$.

What is meant by the power of accommodation of the eye?

$A$ student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is $1 \, m$. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is $25 \, cm$.