Diffraction of Light

Ray (geometrical) optics predicts that an opaque obstacle placed in light's path should cast a perfectly sharp shadow, with light travelling in strictly straight lines right up to the obstacle's edge and nowhere beyond it. In reality, under the right conditions, light is observed to bend somewhat AROUND the edges of obstacles, reaching slightly into regions that geometrical optics would predict should be in complete shadow -- this bending is called DIFFRACTION, and (like interference) it is a phenomenon common to every kind of wave, not just light. Sound diffracts very noticeably in everyday life -- we can hear someone talking from around a corner or through a doorway, even though a solid wall geometrically blocks any direct straight-line path -- precisely because sound's much longer wavelengths make its diffraction easy to observe; light's much SHORTER wavelength means its diffraction, while real, is generally far subtler and only becomes noticeable when the obstacle or aperture size is comparable to that short wavelength. …