How It Works
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What is the smallest thing to cast a shadow?

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Shadows occur when visible light is blocked by an object, leaving a dark area on a surface opposite the source. As light travels as a wave, it bends slightly around objects (diffraction).

Although the light is diffracted by only a few wavelengths, it means any object smaller than the light’s wavelength will have no shadow. Visible light has a wavelength of 400-900 nanometres.

So the tiniest thing to cast a shadow would have a diameter of about 400 nanometres – which is over 200 times finer than a human hair.