Convex Lenses

The pole of the lens is the centre of the lens. Rays drawn passing through this point in an optical diagram are not diverted, they continue in a straight line.

The focal length of a lens is the distance between the pole of the lens and the focal point OR the perpendicular distance between the axis of the lens and the focal plane.

The focal point or principal focus of a convex lens is the point through which rays of light travelling near t,o and parallel to, the principal axis pass after refraction by the lens. (The point all emerging rays pass through).


In the above diagram the refraction of the rays is shown at BOTH lens surfaces.

In the diagram below a SYMBOL for the lens is used - showing the refraction at each surface is then not necessary... it is shown happening at a 'line' - in exam questions make sure you carefully look to see whether symbol or lens drawing is required.

Nowadays an even simpler symbol is used:


The image formed by a convex lens

The type of image you get from a convex lens depends upon the distance the lens is from the object it is viewing.

A convex lens close to the object (a distance less than the focal length) gives a virtual, erect, enlarged image - it works as a magnifying glass.

A convex lens at an infinite distance from an object produces a real (can be picked up on a screen), inverted image. This fact is used in the camera to photograph distant objects.

Lenses focus not only visible light but also infrared. A convex lens focuses the parallel rays of infrared light from the sun into a point focus. It can therefore set things alight! (See the newsreport on a £300,000 fire caused by a glass paperweight)

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