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Lenses

A converging lens is thicker in the middle than at the edges. Rays which hit the lens refract as they enter and again when they leave.

If rays of light parallel to the axis hit the lens, they meet at a single point. The point at which all the rays of light come together is the focal point or the principal focus.

The focal length, $$F$$, is the distance between the centre of the lens and the principal focus.

Light hitting a converging lens
Light hitting a converging lens

Converging lenses are used in eye glasses to enable people with farsightedness to focus on objects better. They are also used in magnifying glasses to make objects appear larger.

A diverging lens is thinner in the middle than at the edges. Rays which hit the lens refract as they enter and again when they leave.

If rays parallel to the axis hit the lens, they are spread out in all directions. The rays of light can be traced back to a point at which they all come together. This is the focal point or the principal focus. The rays do not actually meet.

The focal length, $$F$$, is the distance between the centre of the lens and the principal focus.

Light hitting a diverging lens
Light hitting a diverging lens

Diverging lenses are used in eye glasses to enable people with nearsightedness to focus on objects better. They are also used in cameras to make larger images.