A change in the direction and speed of a light wave passing from one medium to another. As a ray of light passes between the boundaries of the two media, it changes direction and its wavelength can increase or decrease depending on the properties of the two medias.

For example, glass has a higher rate of refraction than air. A ray of light passing through the boundary between air and glass will have its speed and wavelength altered. This effect can be seen visibly when a ray of white light passes through a glass medium. The different waves of light are spread out as they pass .The result is a prism of light.

For profiling clear media, such as glass, refraction can compromise the ability of the colorimeter to give accurate readings as the light leaving the colorimeter is changed when it passes between the boundaries of one media to another. The light scatters instead of reflecting back unaltered.