IB Physics Sub-topic C3 Notes

Single slit diffraction

Remember that diffraction is the spreading of waves after passing through apertures, resulting in an energy distribution in the geometric shadow region.

Single-slit diffraction of a monochromatic source results in the constructive and destructive interference of out of phase waves, called Huygens’ principle. A single-slit diffraction pattern contains a central maximum intensity followed by minima and maxima of decreasing amplitude and intensity, as shown below:

To calculate the angle of the first minimum (θ), based on slit width (b) and wavelength (λ), use the formula:

$sin \theta = \frac{\lambda}{b}$

If the angle is less than 12.5°, small angle approximation can be used, changing the formula to:

$\theta = \frac{\lambda}{b}$

The relative intensities and amplitudes of the first few maxima are outlined in the table below:

Effect of white light

If white light is involved in single-slit diffraction, different wavelengths diffract by different amounts. More specifically, white light forms the central maximum, followed by lower wavelengths having a lower angle of diffraction, and higher wavelengths having a higher angle of diffraction. This results in the distribution shown below: