The wave nature of electromagnetic radiation can most easily be demonstrated in the closely related phenomena of diffraction and interference. When we are dealing with obstacles or apertures much larger than the wavelength, electromagnetic radiation appears to travel in straight lines, subject to the laws of reflection and refraction. The study of electromagnetic radiation under these conditions is called geometric or ray optics. However, when the obstacles or apertures are of order of the same size as the wavelength or smaller, electromagnetic radiation appears to spread out in all directions at the edges. This is called diffraction.

Interference

Like all waves, electromagnetic waves can be made to interfere. If two rays of equal amplitude are in phase (meaning that the phase difference between them is an integer times 360 degrees), they will interfere constructively or reinforce, and the result is a wave of twice the amplitude (four times the intensity) or the original. If they are completely out of phase (phase difference is an integer-and-a- half times 360 degrees), they will interfere destructively or cancel, and the result will be no wave at all. The phase difference depends on path difference. If two waves start out in phase (say from a laser), then travel different paths, there will be a phase difference equal to 360 degrees times the path difference divided by the wavelength. Thus, to sum up:

path difference = integral number of wavelengths Figure 1

path difference = ½ integral number of wavelengths Figure 2