1. Light and 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

3. Diffraction and Interference
Diffraction – spreading of a wave into the area behind an obstacle

4. 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

5. Young’s Double Slit Experiment
Coherent light
2nd order maximum
1st order maximum
Bright central maximum
1st order maximum
2nd order maximum

6. Coherent Light
All waves are in phase
Lasers

7. Pattern seen when light is shined through a double slit at a screen Bright areas are equally bright and equally spaced

8. 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

9. 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:

10. Example
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 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?
d = 8.8 x 10-4 m

11. Single Slit Diffraction
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

12. Diffraction Grating
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