g.c.s.e. OPTICS

Long sightedness

A concave lens is a diverging lens . A light ray passing
through the centre will go straight through whereas
parallel rays on either side spread out (diverge) as if they
come from a point F. (F is the focal point of the lens) the
centre line is called THE PRINCIPLE AXIS

Convex lens

A convex lens is a converging lens. This time the
parallel rays on the outside converge to the focal
point F

A long sighted person can see
objects at a distance but
cannot see close objects. Rays
from a near object focus behind the
retina in the eye (Fig1).

A convex lens helps to rectify
this problem by converging
the rays so that the eye lens can
focus the ray on the retina (Fig 2)

The retina is the light sensitive part at the back of the eye

Short sightedness

So that the eye lens can focus the rays
on the retina. (Fig 4)

Eye

Ray diagrams