# The branches of optics

We can approach the study of light from any of its aspects:

Geometrical optics: the path of light is represented by a ray without reference to either waves or particles.

Physical optics: emphasis on the wave nature of light.

1.5 The nature of light

Newton firmly believed that light was carried by particles (corpuscles). He published his theory of light in the book Optiks (1704). It is ironic that Newton, the firm devotee of the corpuscular theory of light, was the first person known to have observed Newton's rings, which are caused by the interference of light, a wave phenomenon.

At the beginning of the 20th century, a number of experiments appeared to confirm the corpuscular theory. Light had the properties of a particle. Other experiments indicated the opposite. Light behaved as a wave. Reconciling such contradictory evidence appeared to be the main outstanding problem in Natural Philosophy.

### 1.5.2 Light as a wave

Waves can interfere with one another, sometimes reinforcing, sometimes cancelling out. They can also bend around corners. As far back as 1802, Thomas Young had shown that light beams from two thin slits coming together can apparently 'mutually destruct' at certain points. Seen as two waves, the two beams interfere destructively, giving darkness. This can be explained as the crest of one wave superimposing on the trough of another and the two 'cancelling out'. In other places, where two crests or two troughs come together, they interfere constructively and we get an increased intensity — not as dramatic, and easier to accept intuitively. In order to observe the phenomenon, we must arrange that at certain points in space there is always constructive interference

(waves in phase), and at others destructive interference (waves out of phase). The effect is analogous to nodes and antinodes in a vibrating string. When we discuss Young's experiment in more detail (Chapter 8), we will see that the experiment is relatively easy to perform. Suffice it to say that it demonstrates a remarkable effect associated with waves, namely:

light + light = darkness