Diffraction

By Marcus Wilson 18/10/2010 2


If you’re like me, you’ve been mesmerized by the colours created by reflections from a DVD or CD. The discs do a great job of splitting the illuminating ‘white’ light of your home light-bulbs into its constituent colours. But unlike a prism or raindrop, which achieve this effect through refraction (blue light travels more slowly than red through glass, which causes the white light to separate out when it is incident on glass from air), in this case diffraction is the key physical effect.

Diffraction is a property of waves. Water waves diffract, sound waves diffract (this is why, to a large extent, you can ‘hear’ round corners) and light waves diffract. However, with the first two, wavelengths are long (of the order of metres) whereas for light, wavelengths are a bit less than a micrometre (thousandth of a millimetre) in size, and that makes the effects hard to see in everyday life.

Those of you at school will learn about diffraction gratings. Here, you stop the passage of light except for very narrow strips, a fixed distance apart. The result is diffraction – the strips each radiate light, and the waves from each source now interfere. If a peak from one slit always arrives with a peak from another slit, you get constructive interference, and a strong signal (bright light). But if a peak from the wave from one slit arrives at the same momement as a trough from a wave from another, you get destructive interference, and see nothing.

A similar thing happens with reflections from the surface of the DVD. In this case, the DVD has small pits in the surface, about a micrometre in size. This causes strong diffraction of light. But the angles at which you get constructive interference depends on the light’s wavelength, and therefore its colour. Red light, being the longer wavelength, will be diffracted more strongly than blue, and so we see the colours separate out. The effect is a shimmering rainbow of colours on the back of the DVD, thought the mechanism is very different from that which causes the rainbow in the sky.

There’s a nice demo of diffraction with sound at http://phet.colorado.edu/en/simulation/sound  – put up two sound sources and move the listener around to hear the effects of constructive and destructive interference.

There’s also some nice wave effects you can see on Google Earth along the coast near Raglan, though there is some complication with other effects, such as refraction of the wave in shallow water.

 


2 Responses to “Diffraction”

  • So this is more interference of reflected waves from the many different tracks on the CD/DVDs surface than it is diffraction.

    Diffraction requires bending of light (or wave) around a barrier or through a slit.

    • Yes – perhaps the title of the post is badly chosen. (I wrote the title before the rest of the post and didn’t go back to check it afterwards). That said, diffraction and interference are tied up together, and often get used quite loosely by physicists. For example we say ‘diffraction grating’ when ‘interference grating’ is possibly a better terminology. And in the theory of radar reflections from objects (which I used to be involved with in a former life) we use ‘physical theory of diffraction’ to mean something that looks superficially similar to diffraction, but is in fact a different effect; that muddies the waters further. But that’s another blog entry entirely.

      Lipson and Lipson write “Diffraction and interference are sometimes not clearly distinguishable, and different writers attach different meanings to the two words. We shall try to maintain the convention that interference involves the deliberate production of two of more separate beams and that diffraction occurs naturally when a single wave is limited in some way. We shall not always succeed in maintaining this convention…” (S.G. Lipson and H. Lipson, (1981) Optical Physics 2nd edition, Cambridge University Press).

      In that sense then, the colours on a DVD are interference.

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