AQA A Level Physics复习笔记3.4.1 Single Slit Diffraction

• Diffraction is the spreading out of waves when they pass an obstruction
  • This obstruction is typically a narrow slit known as an aperture

   

• The extent of diffraction depends on the width of the gap compared with the wavelength of the waves
  • Diffraction is the most prominent when the width of the slit is approximately equal to the wavelength

   

Diffraction: when waves pass through a narrow gap, they spread out

• Diffraction is usually represented by a wavefront as shown by the vertical lines in the diagram above
• The only property of a wave that changes when it diffracts is its amplitude
  • This is because some energy is dissipated when a wave is diffracted through a gap

   

• The diffraction pattern of light can be represented as a series of light and dark fringes which show the areas of maximum and minimum intensity
• 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 will spread out as follows:

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

• The features of the single slit diffraction pattern are:
  • A central maximum with a high intensity
  • Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum

   

• If the laser were to be replaced by a non-laser source emitting white light:
  • The central maximum would be white
  • All maxima would be composed of a spectrum
  • The shortest wavelength (violet / blue) would appear nearest to the central maximum
  • The longest wavelength (red) would appear furthest from the central maximum
  • The fringe spacing would be smaller and the maxima would be wider

   

Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width

• 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, the effect gradually gets less pronounced until, in the case that the gap is much larger than the wavelength, the waves are no longer spread out

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

• If the blue laser were to be replaced with a red laser:
  • The wavelength of red light is longer so the light would diffract more
  • The intensity fringes would therefore be wider

   

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

• If the slit was made narrower:
  • The intensity would decrease
  • The fringe spacing would be 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