Fig 1.6. shows the principle
of light diffraction. As the wave front travels through the
slit, a new wave front radiates from the slit, this is shown
by the experiment conducted by
If we were to consider light waves as rays, diffraction would
seem to bend them as they pass an edge. Another better understanding
of diffraction would be to pretend that at the points where
the light rays strike the object, a new set of waves is emitted,
which spreads out in all directions, this then brings us to
the point of interference.
Interference is caused by waves
interacting with each other as they intersect, causing a waves
to either add together (constructive interference) or cancel
(destructive interference). When discussing interference, we
are dealing with the electric field.
Fig 1.7. This diagram shows
the phenomenon of light interference. A screen with a double
slit, equally separated, is illuminated with a bright light,
the two slits cause diffraction of the light waves, due to the
presents of two slits, two sets of wave fronts are produced,
eventually the wave fronts will overlap. at the point where
the waves overlap, there will be either Constructive Interference
( bright area ) or Destructive Interference ( dark area ). This
experiment was conducted byThomas Young.
Constructive interference is
the point of intersection between two light waves where their
interaction combine together to give rise to a larger wave (Amplified),
but the wavelength stays the same. The resultant of constructive
interference is a bright band of light.
Fig 1.8. Shows the constructive
interference of two waves, which will form a wave with a larger
This type of interference produces a bright area. The negative
amplitude of one adds to the positive amplitude of the other,
giving rise to an amplified wave.
Destructive interference is the
point of intersection between two light waves where their interaction
will cancel each other out to give a null point with zero amplitude.
Fig 1.9. Shows the destructive
interference of two waves, the two wave cancel each other out
and give rise to a null point with zero amplitude. This type
of interference produces a dark area.
Application of Interference
Interference is a very important
property of light which form the fundamental basis in holography.
In holography, we are actually recording the interference pattern
caused by the intersection of two laser beams. The laser is
split into two beams, one is used to illuminate the object,
and the other becomes the reference beam. Both beams meet at
the holographic plate, where the interaction of both beams setup
an interference pattern. This interference pattern contains
'information' of the object.
Holography will be covered in
a separate tutorial.
Another application of interference
of light is employed in the Acousto-Optic Modulator (AOM). A
transducer produces sound waves which give rise to a standing-wave
in a special crystal. When light interacts with the standing-wave
within the crystal medium, the interference of light waves and
acoustic waves cause diffraction of the incident light.