S-Cool Revision Summary

S-Cool Revision Summary

Diffraction

A wave will diffract (spread out) as it goes through a gap or past an obstacle.

Note: The wavelength remains the same before and after the gap.

Remember this: The nearer the slit size is to the wavelength, the more the wave will diffract.

  1. The smaller the gap the greater the diffraction.
  2. The longer the wavelength the greater the diffraction.

You should be able to describe experiments such as the ripple tank or microwave kit that will show diffraction.

Single Slit Diffraction Pattern

If a wave goes through a slit a diffraction pattern can be detected on the other side, with regions where the wave is intense and regions where the intensity falls to zero. A graph of intensity against distance from the centre of the pattern can be drawn:

Diffraction

Coherence:

Coherent waves are waves with a constant phase difference. (Note: They don't have to be in phase for this to be true.) They will have the same frequency and wavelength (they are normally produced from one source).

Young's Double Slits:

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The pattern formed is of close, bright "fringes" of light.

A bright fringe occurs at P if S2P - S1P = nλ

A dark fringe occurs at P if S2P - S1P = nλ + ½ λ

Interference and Superposition

When two waves meet they will interfere and superpose. After they have passed they return to their original forms. This is true if they are coherent or not.

Path Difference

You will need to be able to work out whether there will be constructive or destructive interference at a point. We do this by comparing how far the two waves have travelled to reach the point. The difference in the distances will tell us if the waves are in phase or not.

Finding the Wavelength of Light

Using Young's double slits to find the wavelength of light:

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λ = wavelength

a = distance between slits

x = fringe spacing

D = distance from slits to screen

Diffraction from a Diffraction Grating

Using a diffraction grating to find the wavelength of light:

nλ = dsinθ

d = slit spacing

θ = angle from centre

n = order of maximum

The pattern that you get with a large number of slits (a diffraction grating) is similar to the double slit pattern in that there are bright fringes on a dark background, but there are far fewer fringes and the gaps between them are much larger.

Double slit pattern:

(closely spaced bright fringes on a dark background)

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Grating pattern:

(widely spaced bright fringes on a dark background)

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