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When I hold my hand underneath my lamp, the shadow my hand casts is crisp and sharp, meaning that the edges are well defined and not blurred. But according to Huygen's principle, shouldn't the light diffract upon reaching the edges of my hands, and perhaps form interference patterns? If so, how can the shadow my hand cast ever be crisp?

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  • $\begingroup$ The short answer (ignoring Huygens principle and thinking of light like waves on a pond) is that frequencies of $10^4$ Hz end up being a totally different ball game than frequencies of $10^{14}$ Hz. $\endgroup$ – user12029 Mar 13 '15 at 23:59
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The shadow of your hand may look crisp to you, but that's because you're not looking closely enough, compared to how short the wavelengths in visible light are.

Indeed, if you look at the shadow from a viewing distance of 40 cm, you can't possibly see the difference between a (hypothetical) exactly crisp edge and one where the intensity of illumination falls off gradually over several hundred wavelengths, due to the angular resolution of your pupils.

Even with magnification, however, you won't usually get diffraction bands in everyday shadows. They just taper off smoothly -- the finite size of the light source will generally dominate over wave effects, or in other words, the shadow consists of superposed shadows cast from different points on the lamp, and their diffraction bands don't match up, so their sum is just a smoothish blur anyway. (This is certainly the case if the shadow is cast by the sun, and also for most artificial light sources).

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The sun makes very sharp shadows as the photons are essentially parallel. An LED lamp can also make shadows appear very sharp, a florescent lamp, not so much. Your experiment ignores the observational limitations (as pointed out by other answers). It's an important point even in theoretical physics.

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    $\begingroup$ Photons are parallel? $\endgroup$ – 299792458 Apr 25 '15 at 7:30

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