Why do stars twinkle?

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

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

The far point of a myopic person is $80 \,cm$ in front of the eye. What is the nature and power of the lens required to correct the problem?

$A$ 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?

$A$ 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?