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AQA AS Level Revision Notes

James Le Cuirot

Combining Waves
Superposition
When two waves of the same type (eg two water waves) at the same point and overlap, the resultant
displacement of the oscillations is the vector sum of displacements of each wave
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Interference
When two waves overlap, they produce regions of constructive and destructive superposition
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Interference:

Coherent:

The name for superposition of waves from two coherent sources of
waves
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Interference is destructive if waves are in antiphase (180° out of phase)
When two waves have fixed phase difference and have the same
frequency

Path Difference
Path difference is the difference in distance travelled by the two waves produced by the sources
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If the waves start in phase they will meet in phase when both waves have travelled the same distance or
where their path difference is a whole number of wavelengths, 𝑛𝜆
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Young’s Slit Equation

AQA AS Level Revision Notes

James Le Cuirot

This is only an approximation and its derivation is such that it only follows for small angles
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Formation of standing wave;

Nodes and Antinodes

Nodes are separated by half a wavelength, as are antinodes
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Harmonics
A harmonic is a mode of vibration that is a multiple of the first harmonic
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Diffraction does not change the wavelength of waves; however, the wave front becomes curved

Diffraction of Light
Single slit diffraction produces an interference pattern
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The interference pattern produced gives a bright central fringe surrounded by two other less bright
fringes on either side
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The dark regions between the maxima are called minima, and are caused by destructive
interference of light
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Diffraction Patterns
Double Slit;

Peaks are same height and same width
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Diffraction Gratings
A diffraction grating has thousands of slits spaced very closed together
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The slits are very narrow and so light is diffracted through a very large angle
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This central maxima is called the 0th Order because there is 0 path difference
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The condition for constructive interference is that the path difference for waves leaving
adjacent slits must be a whole number of wavelengths
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For the 2nd order, the path difference
is two wavelengths and so on
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mm (1×10−3) divided by the number of lines per mm]

Application of Diffraction Gratings
Diffraction gratings are used in telescopes to analyse light from galaxies allowing us to look at the
elements that make-up that galaxy
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The light source
would be a laser installed in the CD / DVD drive in a computer or DVD player

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