As light travels from one medium to another. its frequency does not change but its wavelength does. To see why this is so. Consider figure waves pass an observer at point A in medium I with a certain frequency and are incident on the boundary between medium I and medium 2.The frequency with which the waver pass an observer at point B in medium 2 must equal the frequency at which they pass point A . If this were not the case . Then energy would be piling up at the boundary . Because there is no mechanism for this to occur. The frequency must be a constant as a light ray passes from.one medium into another . Therefore. because the relationship