In the figure you can see a top view of a water tank in which waves are produced at the left side and travel from left to right. The bright and dark bands in the photograph represent crests and valley of the waves; thus adjacent bright band are one wavelength apart. The waves encounter a barrier into which a small hole has been cut. You can see how the waves flare out after they pass through the hole and appear in the region behind the solid barrier. This phenomenon is known as diffraction. Diffraction, which we discuss in detail in any other question Occurs whenever a wave encounters an opening whose dimensions are roughly of the same size as the waves length of the wave. In the case size of the opening is about the same as the wavelength. The flaring effect decreases as the size of the opening is made larger than the wavelength. If the size of the opening is any times the wavelength, the effects of diffraction are negligible, and after passing through the opening the wave will appear only in the geometrical shadow of the opening. Similar effects occur if the wave encounters an obstacle instead of an opening in a barrier.

The wavelength of visible light is in the range of 400 to 700 nm. Even a tiny mirror or lens or diameter only a few mm is 4 orders of magnitude larger than the wavelength of light. Diffraction effects are thus almost always negligible in the formation of images by mirrors and lenses. This is the realm of geometrical optics, in which light beams encounter objects that are much larger than the wavelength of the light. The conditions of geometrical optics will apply in all cases.

In geometrical optics we can analyze the formation of images by assuming the light to travel in straight line paths or rays. For this reason, geometrical optics is also known as ray optics. A ray is a convenient way of describing the travel of light; rays are drawn perpendicular to the wave.

On the other hand, if diffraction effects occur, we are in the realm of physical optics, also known as wave optics because we must take the wave nature of light specifically into account in the analysis. Interference and diffraction are examples of effects for which we must use the methods of physical optics.