Ordinarily light travels in straight lines through a uniform, transparent medium, but those light waves that just pass the edges of the opaque body are bent, or deflected. This diffraction produces a fuzzy border region between the shadow area and the lighted area. Upon close examination it can be seen that this border region is actually a series of alternate dark and light lines extending both slightly into the shadow area and slightly into the lighted area. If the observer looks for these patterns, he will find that they are not always sharp. However a sharp pattern can be produced if a single, distant light source, or a point light source, is used to cast a shadow behind an opaque body.
Diffraction also occurs when light waves interact with a device called a diffraction grating.
In the case of a reflection grating, the smooth surfaces between the lines act as narrow slits. The ruling is generally done with a fine diamond point. The diffraction of X rays by crystals is used to examine the atomic and molecular structure of these crystals. Beams of particles can also exhibit diffraction since, according to the quantum theory , a moving particle also has certain wavelike properties. Both electron diffraction and neutron diffraction have been important in modern physics research.