Diffraction 2
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Transcript Diffraction 2
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