The change in focal length of an eye lens is caused by the action of the

$A$ person needs a lens of power $-5.5 \, D$ for correcting his distant vision. For correcting his near vision,he needs a lens of power $+1.5 \, D$. What is the focal length (in $cm$) of the lens required for correcting near vision?

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

Explain why the planets do not twinkle.

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

$A$ 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?