Interference

The final property of light we discuss is interference, a phenomenon that occurs when two light beams meet. Depending on the nature of the light beams and when they meet, the two beams might enhance each other, to give a brighter beam, or they might interfere in a way that makes the total beam less bright. The former is called constructive interference, whereas the latter is called destructive interference. Also for reasons we shall see later, we don't often encounter interference of light in our everyday lives. One situation that you might be familiar with is when there is a thin film of oil floating on the surface of a lake, for example. Sometimes the oil will display a brilliant pattern of colours, even when illuminated by white light. What is happening is that different parts of the oil film cause the different colours in the white light to interfere constructively or destructively, depending on the thickness of the film at that point. For example, if one region of the film might look red because the red light bouncing off the top of the oil interferes constructively with red light that passes through the oil and reflects back off of the water below it.

We encounter interference more commonly in the context of sound: Consider sitting in a (poorly designed) auditorium at the point ``S'', as below.

 

Figure 10.10: Interference effects

Sounds from the stage could reach you from several different paths: a direct path, or reflected from the ceiling, walls, or floor. At some points one might find a ``dead zone'', where one cannot hear anything - this is where the sound coming from all these different paths cancel each other out. At some other points the sound might be abnormally large - this is where the sounds coming from all the different paths add together. In such cases the sound exhibits interference, which can be either destructive or constructive. This is the reason modern auditoriums are designed with sound-absorbing walls, ceilings, and floors.

An important experimental situation in which light signals can be observed to interfere with each other is the following. Suppose identical light rays from the same source are diffracted by two slits and meet at a distance screen, as in the figure below. (This is called a Young's double slit experiment, after the physicist who first used it to demonstrate the interference of light.

  

Figure 10.11: Double slit interference

What is observed is that there are alternate dark and bright spots on the screen, which correspond to points where light rays coming from the two different slits undergo destructive and constructive interference, respectively. Note that the light rays are not interacting with each other in the usual sense: one ray does not exert a force on the other. This sort of interference can only be explained by thinking of light as waves, as we will now discuss.