Single Slit and Non-Monochromatic (Diffraction and Interference) – AS Physics Revision

Single Slit

Single slit makes diffraction pattern not sing...

Single slit makes diffraction pattern not single band; double slit makes interference pattern. Same basic apparatus. (Photo credit: Wikipedia)

  • When you diffract through a single slit, the light still spreads out and interference still happens, but the equation works differently
  • Please ensure you’ve read the paragraph about equations for multiple slit diffraction in my other slit diffraction and interference post
  • You still write it as n λ = d Sin(θ), but…
  • Firstly, the angle tells you where the dark fringes are (previously it was telling us where the bright ones were)
  • Secondly, d in the equation represents the slit width, not slit separation
  • (The way to remember that one is to learn that d is slit separation by default, but then come to a single slit diffraction question and think “what do you mean, slit separation? There’s only one slit here. Oh, yeah – with single slit diffraction, d is width, not separation!”)

A Spreading Beam

  • A beam of light that has been squeezed through a (single) slit will spread out further and further the further it goes
  • (Well, unless you put a screen in the way)
  • A beam spreads away from the horizontal/straight through line at angle θ, which, in this case, must be in radians, and after travelling L metres from the slit, is w metres wide
  • θ = λ/d which is roughly equal to w/L, because theta is such a small angle
  • This ‘width’ is from the middle of the beam to one of the edges, though, so most people would consider it to be half the width of the beam. If you want the width of the beam from the edge to the other edge, double w

Multichromatic (well, as opposed to monochromatic. Multi-chromatic isn’t a word)

  • If you shine white light through a diffraction grating (or double slit), you get a spectrum. The wavelengths that make up white light all diffract differently, so they spread out into a rainbow
  • I’m not sure whether the following bit is on the syllabus, or not. This is physics, not chemistry, after all. I’m not sure whether that way of thinking about atoms is still seen as correct, either…
  • Electrons can be excited – given more energy than they’d usually have – causing them to hop up into a higher energy level / electron shell
  • Eventually, the excitement wears off, and they return to their ground state / normal electron shell
  • When this happens, the energy has to go somewhere, so it’s released as a photon
  • The energy / frequency / wavelength of this photon depends on the electron shells the electron jumped between
  • Studying that photon would reveal information about the atom
  • Different elements have different spectra (it’s to do with their electron shell energies)
  • Sodium light (the light emitted by an excited sodium ion) is bright yellow, and right in the middle of the visible spectrum
  • Helium gets its name from the sun, which is where it was first found, by analysing the spectrum of sunlight

One More Thing

  • If I’ve been typing “slot” instead of “slit” in any of these posts, I apologise. Those words are too similar!
  • They mean roughly the same thing, and the ‘i’ key is right next to the ‘o’ key on a QWERTY keyboard
  • The one more thing is that light diffracts at edges, as well as slits – it can diffract round a wall, or a mountain, sort of. It would have to have a big wavelength to do so, though.
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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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