Up: Properties of light
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.
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.
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.
Up: Properties of light