The Huygens-Fresnel principle tells us to pretend that each point of a wavefront in a slit or aperture is a point source of light emitting a spherical wave. Is this true only for points inside the slit? What if there is no slit? The Huygens-Fresnel principle really applies 1. to any point anywhere in a beam path. 2. to any point in a beam path where matter is present. 3. only in slits or apertures.