Light and diffraction

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 and Interference 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

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

Coherent Light 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:

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 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? d = 8.8 x 10-4 m

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

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