Slitto

Meet Slitto, a wall with one narrow gap who spreads light out the narrower he makes it.

When light passes through a slit only a little wider than its wavelength, it does not just go straight. Waves from different parts of the gap interfere, and the light fans out into a broad bright band with fainter bands on either side.

The dark gaps appear where a sinθ = mλ, with m = 1, 2, 3 and so on. For a slit 0.1 mm wide and green light of 500 nm, sinθ = 0.005 for the first dark band. On a screen 1 m away that is 5 mm from the centre, so the central bright band is about 10 mm wide.

Halve the slit width and the band doubles. That surprising rule is a clear sign that light behaves as a wave.

Diffraction like this also limits how sharp any camera, microscope or telescope can be, because every opening spreads the light a little.