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Hold two cards (say credit cards) edge to edge, anything from a very slight touch to about 1/3 mm separation, in front of any ordinary light source. When I do this I see several fine dark parallel lines in the gap. What are those? Are they discussed in elementary texts?

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Interference fringes en.wikipedia.org/wiki/Interference_%28wave_propagation%29 . You can see them between two fingers too.Reflected light interfering with the source light. –  anna v Feb 29 '12 at 16:27
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@annav reflected light? I thought that the different bits of diffracted light interfere. O_o –  Manishearth Feb 29 '12 at 16:43
    
@Manishearth right, I saw it as grazing reflection, which is diffraction. That is how diffraction. gratings work after all –  anna v Feb 29 '12 at 18:32
    
Diffraction and interference are the same phenomenon. If you like, you can call diffraction the interference of a beam with itself. –  Colin K Feb 29 '12 at 20:14
    
@Manishearth The hyperphysics reference explains single wavelength very well. How does one quantify the result when my light comes from, say, gray clouds? –  Bob Terrell Feb 29 '12 at 20:36
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This is just an addendum to the excellent answer already given. The easily observed existence of such diffraction patterns is the main reason why by the start of the 1900s everyone "knew" that light had to consist entirely of waves. Thus it was a bit of a shock to everyone when in 1905 an undistinguished non-academic published a little paper that showed a remarkable implication of certain experiments in combination with some earlier postulates by a physicist named Max Planck.

His idea was that light must form discrete units with well-defined energies. The energies of these decidedly non-wave packets depended only on their color (frequency), and had to be emitted or absorbed in whole units, very much like particles. This was so unexpected that it took many years for physicists to accept such an utterly counter-intuitive idea, which flew in the face of even such trivial observations like the one you just mentioned using two ordinary playing cards. Even Planck initially rejected the idea, saying that it would set the understanding of light back by centuries.

When this bizarre and seemingly contradictory prediction came to be widely accepted years later, its originator earned a Nobel Prize for it. But far more importantly, that acceptance helped trigger one of the most amazing periods of fast-paced intellectual exploration in the history of the world: The development of modern quantum mechanics in the mid 1920s. It was a time of exploration so revolutionary and unexpected that even special and general relativity seemed almost mundane by comparison.

Not that the originator of the 1905 paper on packets of light cared much. That's because he happened to be the same person who developed special and general relativity. That's right: Albert Einstein earned his one and only Nobel Prize -- he should have earned about three more, really -- not for relativity as one might suppose, but for triggering the development of modern quantum mechanics. Now that was a remarkable mind!

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They're diffraction fringes. Whenever light or any other wave hits a corner/obstaclr, it has the tendency to bend around it. Now, the waves bend by varying amounts, so we get a phenomenon called interference. Some waves 'add up' and give slightly brighter lines, other waves 'cancel out' to give a dark fringe.

You can do this experiment with two fingers as well.

Here's an explanation of diffraction: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslitd.html#c1

If the link is too technical, let me know, and I'll expand my answer. (i haven't really explained interference)

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