Single Slit Diffraction (AQA AS Physics)

• Diffraction is:

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

• • When waves meet a narrow gap they must curve to pass through
• The extent of their diffraction depends on the width of the gap compared to the wavelength of the waves

Diffraction: after passing through a narrow gap, the waves curve as they spread out

• The wavefronts of the wave represent the crests and troughs
  • The diagram shows what happens to the wavefronts as they pass through the narrow gap

Wavefronts and rays for transverse waves travelling in a horizontal plane

• The only property of a wave that changes when it diffracts is its amplitude
  • The wavelength of the wave remains the same
• The amplitude of the diffracted waves is less than that of the incident waves since energy is distributed over a larger area
• Imagine trying to squeeze yourself through a narrow gap. How do you do it? 
  • You make yourself narrower
  • A wavefront will do the same
• The greater the wavelength of the wave, the greater its diffraction
• Examples of diffraction include:
  • Radio waves moving in between or around buildings
  • Water waves moving through a gap into a harbour

Single Slit Monochromatic Diffraction Pattern

• The diffraction pattern of light passing through a single slit, like a double slit, is a series of light and dark fringes on a far away screen
  • The bright fringes are also 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 also areas of zero or minimum intensity, produced by the destructive interference of each part of the wavefront as it passes through the slit

Single Slit Monochromatic Intensity Pattern

• If a laser emitting blue light is directed at a single slit, where the slit width is larger than the wavelength of the light, it's intensity pattern will be as follows:

The intensity pattern of blue laser light diffracted through a single slit

• The features of the single slit diffraction 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 the following diffraction pattern:
  • The diffraction pattern obtained is similar but slightly different to that of a double slit
  • The central maximum would be white. It is wider and brighter than in the double slit diffraction pattern
  • There are no dark destructive interference fringes either side of this central maximum, like in the double slit pattern
  • All maxima are also 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
  • The longest wavelength (red) would appear furthest from the central maximum
  • 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
  • The fringe spacing would be smaller and the non-central maxima would be wider

The diffraction pattern of white light diffracted through a single slit

• 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

The intensity pattern for the diffraction of white light through a single slit

• As discussed above, the effects of diffraction are most prominent when the gap size is approximately the same or smaller than the wavelength of the wave
• As the gap size increases, compared to the wavelength, the amount of curvature on the waves gets less pronounced
  • This is because the wave has to curve less to fit through the gap
• When the gap is much larger than the wavelength then it no longer curves and no longer spreads out after passing through the gap

The size of the gap (compared to the wavelength) affects how much the waves spread out when diffracted through a 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

Fringe width depends on the wavelength of the light 

• 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

The intensity pattern of red laser light shows longer wavelengths diffract more than shorter wavelengths

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

     ANSWER:   D

• Diffraction is most prominent when the wavelength is close to the aperture size
• UV waves have a wavelength between 4 × 10^–7 – 1 × 10^–8 m so won’t be diffracted by a gate post
• Sound waves have a wavelength of 1.72 × 10^–2 – 17 m so would not be diffracted by the diffraction grating
• Radio waves have a wavelength of 0.1 – 10⁶ m so would not be diffracted by human hair
• X-rays have a wavelength of 1 × 10^–8 – 4 × 10^–13 m which is roughly the gap between atoms in a crystalline solid
  • Therefore, the correct answer is D