Waves can be added together either constructively or destructively. The result of
adding two waves of the same frequency depends on the value of the phase of the
wave at the point in which the waves are added.
Electromagnetic waves are subject to interference. For two sources of electromagnetic
waves to interfere:
- the sources must have the same frequency and polarization,
- the sources must be coherent, and
- the superposition principle must apply.
In the double slit experiment, a single source is split in two, to generate two
coherent sources. When the light from the two sources is projected on a screen,
an interference pattern is observed.
To explain the origin of the interference pattern, consider the distance traveled
from the two sources. At the center of the screen the waves from the two sources
are in phase. As we move away from the center, the path traveled by the light from
one source is larger than that traveled by the light from the other source. When
the difference in path is equal to half a wavelength, destructive interference occurs.
Instead, when the difference in path length is equal to a wavelength, constructive
This is a classic example of interference effects in light waves. Two light rays
pass through two slits, separated by a distance d and strike a screen a distance,
L, from the slits. You can change this parameters (drag scrollbars to do
it) and you see the result of interference on the screen.