diffraction

• diffraction is the spreading out of waves when they pass an obstruction or pass through a narrow gap.
• the amount of diffraction depends on the wavelength of the wave compared with the size of the gap:

  • when the gap is a lot bigger than the wavelength, diffraction is unnoticeable
  • you get noticeable diffraction through a gap several wavelengths wide
  • you get the most diffraction when the gap is the same size as the wavelength
  • if the gap is smaller than the wavelength, the waves are mostly reflected back
• when sound passes through a doorway, the size of gap and wavelength are usually roughly equal so lots of diffraction occurs, which is why you can hear someone in another room.

  however, when light passes through a doorway, the size of the gap is a lot bigger than the wavelength so there is hardly any diffraction, so you can't see someone in another room
• to observe a clear diffraction pattern for light, you need to use a monochromatic, coherent light source.

  • monochromatic = all the light has the same wavelength

  lasers are an example of a monochromatic and coherent light source
• you will see a central bright fringe called the central maximum, as well as dark and bright fringes alternating on either side. the dark fringes are caused by destructive interference and the bright fringes are caused by constructive interference

  
• white light is a mixture of different colours, each with different wavelengths. when white light is shone through a single narrow slit, all the different wavelengths are diffracted by different amounts. this means that instead of clear fringes, you get a spectra of colours with dark fringes, and a white central maximum

  
• the central maximum in a single slit light diffraction pattern is the brightest part of the pattern. this is because the intensity of light is highest in the centre.

  • intensity is the power per unit area

  for monochromatic light, all photons have the same energy, so an increase in the intensity means an increase in the number of photons per second. this means there are more photons per unit area hitting the central maximum per second than the other bright fringes
• the width of the central maximum varies with wavelength and slit size:

  • increasing the slit width decreases the amount of diffraction. this means the central maximum is narrower, and the intensity of the central maximum is higher
  • increasing the wavelength increases the amount of diffraction. this means the central maximum is wider, and the intensity of the central maximum is lower