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

Give one cause of near-sightedness (myopia).

What is the central aperture of the eye located behind the cornea called?

$A$ student cannot read properly from the blackboard while sitting on the front desk in a large classroom. However,he can read clearly while sitting on the last desk of the classroom.
$(a)$ Draw ray diagrams to illustrate the formation of the image of the blackboard writing by his eye lens when he is seated at the $(i)$ front desk,$(ii)$ last desk.
$(b)$ Name the defect of the eye from which the student is suffering.
$(c)$ Name the type of lens that would enable him to see the blackboard writing clearly when seated at the front desk. Draw a ray diagram to illustrate how this lens helps him to see clearly.

The focal length of the eye lens can be changed due to the action of $......................$.

Which lens is used to correct the defect of hypermetropia (farsightedness)?