# Section 5: Geometrical Optics

Geometrical optics marks the transition to the description of light not via waves, but rays. Indeed, the majority of the time the wave picture of light is not necessary, which is why most introductory classes in optics use ray descriptions with almost no loss of accuracy. You may be asking, well then why did we learn the wave picture at all?! I hear you and understand your fury, but it is important to keep in mind the contexts in which the ray picture can be used. The ray picture comes into play in the limit that the wavelength of light goes to zero, which one may imagine would occur when the wavelength is dwarfed by another quantity. In this case, you would be correct! The ray picture is satisfactory when the size of the objects the light is interacting with is much larger than the wavelength. Thus, the wave picture is necessary when talking about how waves interact with atoms to produce reflection/transmission, but it is sufficient to determine the direction of light propagation in a laser setup using rays because the sizes of lenses, mirrors, etc. are much larger than the wavelength of light. Rays are drawn perpendicular to the planar wave fronts of the wave picture, and thus can be thought of as parallel to the $k$-vector. This section will be much easier than the previous lessons we’ve seen, so look forward to that!