Christian Huygen’s Principle (1678)


One wave front is made up of many small
wave fronts
Most of the small wave fronts cancel out
because of interference

What is left are the larger wave fronts that we see

Diffraction – spreading of a wave into the area
behind an obstacle
In the 18th century, Thomas Young proved that light
waves interfere
 Until this time there was a lot of controversy about
whether light was a wave or something else
For interference patterns to be seen, light must pass
through narrow slits
 Young’s experiment involved two slits placed very
close together
Coherent light
2nd order maximum
1st order maximum
Bright central maximum
1st order maximum
2nd order maximum

All waves are in
phase

Lasers
Pattern seen when light is shined through a double
slit at a screen
Bright areas are equally bright and equally spaced
This is a simplified version of double slit diffraction
 Where a light fringe occurs there is constructive
interference, either two crests line up or two troughs line up
 Where a dark fringe occurs there is destructive interference,
a crest and trough line up
Distance between bright spots, x,
is related to:
d = distance between slits
L = distance from slits to screen
λ = wavelength of light
by the equation: x
L


d
Judy and Earl are sitting under the boardwalk on a warm
summer evening while the light of a low pressure sodium
vapor lamp whose wavelength is 589 nm passes through
two small cracks in a board, producing fringes of light
0.0020 m apart on the ground
a) If the boardwalk is 3.0 m above the sand, what is the
distance between the two cracks in the board?
x 

L d
0.0020m 589 x10 9 m

3.0m
d
d = 8.8 x 10-4 m
Even when light passes through one narrow slit
an interference pattern can be seen
Brightness decreases from the center
Width of bright areas decreases from the center

Piece of film with many very fine line scratched
into it



i.e 10,000 lines/cm
More slits means larger distance between
bright spots (can be > 1m apart)
Used to measure wavelength of light because
distance between bright spots is easier to
measure
Double slit
Five slits