An understanding of both geometric and physical optics allows us to study phenomena resulting from both particle and wave aspects of light.

Light travels through space as electromagnetic waves and as particles. As a result of this particle-wave duality, when physicists are working with optics (the study of light), they must think of the propagation of light in one of two ways, depending on the application.

When thinking about such characteristics of light as interference, polarization or color, describing light as transverse wave fronts is the way to go. But when building a telescope or corrective lens and determining how light will reflect, refract and transmit, the best option is to think of light as a beam of particles moving in straight lines called rays.

The study of physical optics uses the wave nature of light to understand such phenomena as interference patterns caused by light waves passing through diffraction gratings and spectroscopy. Physical optics took off as a field in the 1800s after several key discoveries, including the existence of light outside the visible spectrum by Sir Frederick William Herschel.

In physical optics, light is represented as a transverse wave front, like the sinusoidal or "S-curve" that also describes a wave traveling through the water with crests and troughs (high and low points). With this model, light waves follow the same rules as other transverse waves – their frequencies and wavelengths are inversely proportional due to the wave speed equation, and the wave fronts interfere with one another where they intersect.

For example, two crests (high points) or two troughs (low points) that overlap interfere constructively, making the overall crest higher or the overall trough lower, respectively. Where the wave fronts meet out of phase – a crest and a trough together – they interfere destructively, either fully or partially cancelling each other out.

Thinking of light as a wave is also key to understanding the differences between types of light in the electromagnetic spectrum, such as the difference between radio, visible and x-rays, since those types are classified by their wave properties. This also means treating light as a wave is important in the physical optics of color, since that is a subset of the visible portion of the spectrum.

In geometric optics, physicists use the particle nature of light to represent its path in straight lines known as rays. Geometric optics has been in use for far longer than physical optics, as people had learned how to design devices that bend and focus light for purposes such as making telescopes and corrective lenses well before they understood what light was. By 1600, grinding lenses for the purpose of aiding human vision was commonplace.

Light rays are drawn as a straight lines emanating from a light source and indicating the direction that light travels. A ray diagram is used to show the paths of several representative light rays as they reflect, refract and transmit through different materials in order to determine such measurements as focal length and the size and orientation of the resulting image.

By tracing the paths of rays of light, physicists can better understand optical systems including image formation in thin lenses and plane mirrors, optical fibers and other optical instruments. Given its long history as a field, geometric optics has led to several well-known laws about how light bounces and bends, perhaps most famously the law of refraction (Snell's law) and the law of reflection.