Single Slit Diffraction (AQA A Level Physics): Revision Note

Diffraction

• Diffraction is:

The spreading out of waves after they pass through a narrow gap or around an obstruction

• Diffraction effects are most noticeable when the wavelength of the wave and the width of the gap are similar in size

• For light, this effect becomes significant with very narrow slits, such as those used in single or double slit experiments

Single slit diffraction pattern

• The diffraction pattern of light passing through a single slit is a series of light and dark fringes on a screen

  • The bright fringes are areas of maximum intensity, produced by the constructive interference of each part of the wavefront as it passes through the slit

  • The dark fringes are areas of zero or minimum intensity, produced by the destructive interference of each part of the wavefront as it passes through the slit

• The central maximum is:

  • much wider and brighter than the other bright fringes

  • much wider than that of the double-slit diffraction pattern

• On either side of the wide central maxima are much narrower and less bright maxima

  • These get dimmer as the order increases

Single slit intensity pattern

• If a laser emitting blue light is directed at a single slit, where the slit width is similar in size to the wavelength of the light, its intensity pattern will be as follows:

• The features of the single slit intensity pattern are: 

  • The central bright fringe has the greatest intensity of any fringe and is called the central maximum

  • The dark fringes are regions with zero intensity

  • Moving away from the central maxima either side, the intensity of each bright fringe gets less

Single slit diffraction and intensity patterns of white light

• A source of white light diffracted through a single slit will produce a diffraction pattern which is different to that produced by a double slit (or a diffraction grating)

• The central maximum is bright white because constructive interference from all the colours happens here:

  • Much wider and brighter than the other bright fringes

  • Much wider than that of the double-slit diffraction pattern

• All other maxima are composed of a spectrum

• Separate diffraction patterns can be observed for each wavelength of light

  • The shortest wavelength (violet / blue) would appear nearest to the central maximum because it is diffracted less

  • The longest wavelength (red) would appear furthest from the central maximum because it is diffracted more

• The colours look blurry, and further away from the central maximum, the fringe spacing gets so small that the spectra eventually merge without any space between them

  • As the maxima move further away from the central maximum, the wavelengths of blue observed decrease and the wavelengths of red observed increase

• A source of white light diffracted through a single slit will produce the following intensity pattern:

  • The central maxima is equal in intensity to that of monochromatic light

  • The non-central maxima are wider and less intense

  • The fringe spacing between the maxima get smaller

  • The amount of red wavelengths in the pattern increases with increasing maxima, n increases from n = 1, 2, 3...

  • The amount of blue wavelengths decrease with increasing maxima

Single slit diffraction and Young's fringes

• In Young's double slit experiment, light passing through the two slits interfere, but each slit also diffracts light like a single slit

• The double-slit interference pattern has equally spaced intensity peaks with maxima of equal intensity

• The single-slit intensity pattern has a distinctive central maximum and subsequent maxima at lower intensity

• Together, the combined intensity pattern has equally spaced bright fringes but now within a single slit 'envelope' 

Single slit diffraction

• As discussed above, the effects of diffraction are most prominent when the gap size is approximately the same as the wavelength of the wave

  • As the gap size increases, compared to the wavelength, the waves spread out less after they pass through the gap

Changes in wavelength

• When the wavelength passing through the gap is increased, then the wave diffracts more

• This increases the angle of diffraction of the waves as they pass through the slit

  • So the width of the bright maxima is also increased

• Red light, which has the longest wavelength of visible light, will produce a diffraction pattern with wide fringes

• Blue light, which has a much shorter wavelength, will produce a diffraction pattern with narrow fringes

• If the blue laser is replaced with a red laser:

  • There is more diffraction as the waves pass through the single slit

  • So the fringes in the intensity pattern would therefore be wider

Changes in slit width

• If the slit was made narrower:

  • The angle of diffraction is greater

  • So, the waves spread out more beyond the slit

• The intensity graph will show that: 

  • the intensity of the maxima decreases

  • the width of the central maxima increases

  • the spacing between fringes is wider