Photoelectric Effect, Photomultipliers and Resolution – AS Physics Revision

 

 

Photoelectric effect

Photoelectric effect (Photo credit: Wikipedia)

Photoelectric Effect

  • The photoelectric effect is one of the pieces of evidence which proves that light isn’t just a wave
  • Photons (the quanta of light. I don’t think they need introduction) transfer their energy to an electron, when they hit one
  • If an electron gains enough energy to break its bond with the atom, it will go flying off
  • The electrons furthest from the nucleus are the ones that have the most energy to start with
  • Because of relationship between energy and frequency of electrons (featuring the Planck constant) some frequencies/wavelengths/colours of light or electromagnetic waves work better for the photoelectric effect than others
  • The wave theory could have predicted that the light intensity would dictate the number of electrons emitted, but it couldn’t explain that it actually depended on the frequency, not the intensity
  • The photoelectric effect works best on a low work function (think of it as electron escape velocity), negatively charged (so there are plenty of electrons), metal plate, using high-energy (high-frequency) photons

Photomultipliers

  • Photomultiplies (a type of vacuum phototube, according to Wikipedia) are devices that allow you to detect tiny amounts of light, by ‘amplifying’ a single photon
  • They’re used in medical imaging, nuclear/particle physics and astronomy (you can tell I had homework to do on this)
  • (Apparently, they’re also part of radar jamming and night-vision goggles. Oh, Wikipedia!)
  • They rely on the photoelectric effect to work, and also something called secondary emission
  • Secondary emission is when metal surfaces are hit by a beam of electrons, and emit more electrons than actually hit them
  • Here’s what happens… (in an ordered list, needlessly…) <l>

Detecting Photons:

  1. A photon arrives
  2. It hits a negatively charged metal plate with a low work function (see photoelectric effect paragraph)
  3. An electron is excited, and leaves the plate
  4. It hits another metal plate, where secondary emission takes place
  5. The electrons hit another metal plate, where secondary emission takes place
  6. The electrons keep hitting metal plates, and secondary emission keeps taking place
  7. Each time secondary emission happens, the electron beam has more electrons in it
  8. Eventually, the beam is big enough, and it lands on a sensor, which informs a scientist that a photon has been detected

Resolution

  • The resolution of a sensor is the smallest change in input it is able to detect (and produce a measurable change in output for)
  • When you’re talking about interference patterns, it’s more to do with how close two patterns can be before they’re indistinguishable
  • i.e. are they far enough apart that you can tell one from the other?
  • The closest two interference patterns can get before they’re indistinguishable is when the first minimum of one lines up with the peak of the other
  • To improve the resolution, you could increase the slit width, or decrease the wavelength of the wave (like they did when they switched from red ray to blu-ray, to fit more information on a disk)
  • I seem to have written an equation in my book. I’m not sure quite what it’s for, but it’s next to the stuff on resolution. Here it is:
  • θ = width/L
  • I might work out what that means and get back to you…
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About Matt

I like writing, filmmaking, programming and gaming, and prefer creating media to consuming it. On the topic of consumption, I'm also a big fan of eating.
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