What is the smallest thing to cast a shadow?
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.