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

Bright central maximum

1st order maximum

2nd order 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 slitsL = distance from slits to screenλ = wavelength of light

by the equation: dL

x

ExampleJudy 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 grounda) If the boardwalk is 3.0 m above the sand, what is

the distance between the two cracks in the board?

dL

x

d

mx

m

m 910589

0.3

0020.0

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 centerWidth 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