In the 17^th century Newton (Newton biography) thought that light was made up of corpuscles; these corpuscles, he imagined, were like tiny balls travelling very fast.

Huygens (Huygens biography), a Dutch physicist and mathematician, was convinced  that light had a wave-form.

Nowadays, we consider that light consists of packets of energy in a wave-form called photons; so in fact both Newton and Huygens were correct.

These photons travel very, very fast (the speed of light) – 3 x 10⁸ m/s.

The wave-form of light can explain refraction and diffraction, as well as other phenomena of light.

In the following section on lenses and optical instruments, we will consider ray optics. A ray of light is represented by a line and an arrow on it, emanating from the light source.

At the end of this section you will understand :

• how to find the position and size of an image formed by a convex lens by constructing ray diagrams.

• how to calculate image distances and sizes formed by a convex lens, using the lens formula.

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