FORM 4: 5.4 UNDERSTANDING LENSES

CONVEX LENSES

A convex lens is thicker at its centre than at the edges.

CONCAVE LENSES

A concave lens is thinner at its centre than at the edges.

FOCAL POINT AND FOCAL LENGTH OF A LENS

1. Focal point, F is the point where the rays converge to it after passing through the lens.
2. Focal length, f is the distance between the focal point and the centre of the lens.

THE MEANING OF POWER OF LENSES

The power of a lens is defined as the ability to bend a light ray entering it.

• The higher the power of a lens, the shorter its focal length and vice versa.
• The thicker the centre of a convex lens, the shorter the focal length of the lens, the higher the power of the lens.

RAY DIAGRAM OF THE IMAGE FORMED BY A CONVEX LENS

RAY DIAGRAM OF THE IMAGE FORMED BY A CONCAVE LENS

MAGNIFICATION, m

LENS FORMULA

THE USE OF LENSES IN OPTICAL DEVICES