Breathe on a glass plate, then sprinkle a fine layer of lycopodium powder onto the plate. Light from a bright point source is focused by a 50 cm focal length convex lens into a hole in the front of a box, and the aperture is adjusted so that the light (without the scattering powder plate) just passes through the hole. When the powder plate is positioned in the beam, the diffraction is sufficient to create a colored halo around the hole, as pictured above. Using the average size of the lycopodium powder spheres (about 25-40 microns) the diffraction angle can be calculated approximately.
Actually this effect is a GLORY rather than a HALO; a halo is a refraction/dispersion phenomenon while a glory is a diffraction phenomenon.
There is a light diffraction ring with a diameter larger than the central scattering halo that is hard to see in the photograph above but which is readily apparent in the real thing.