64 The Ray Aspect of Light

Light can travel from a source to an observer in several ways. It may travel directly through empty space, as sunlight does on its way from the Sun to Earth. It may pass through one or more transparent materials, such as air, water, glass, or the tissues of the eye. Light may also reach an observer after reflecting from an object, such as a mirror or the surface of the skin.

In each of these situations, light can often be represented as traveling along straight lines called rays. A ray indicates the direction in which light travels. Light may change direction when it reflects from a surface or passes from one material into another, but between these interactions it travels approximately in a straight line.

The ray model works well when light interacts with objects that are much larger than its wavelength. Visible-light wavelengths are approximately 400 to 700 nm, much smaller than most objects encountered in everyday life. Therefore, when light interacts with mirrors, lenses, windows, the cornea, or other visible objects, its path can usually be treated as a straight ray.

The ray model does not describe every property of light. When light encounters very small openings, edges, or structures comparable in size to its wavelength, wave effects such as diffraction and interference become important. Those effects will be studied later in wave optics.

Geometric Optics

The branch of optics that treats light as traveling along rays is called geometric optics. Because rays are represented by straight lines, many optical problems can be solved using geometry and trigonometry.

Two fundamental processes determine how light changes direction when it interacts with matter:

• Reflection occurs when light strikes a surface and returns to the original medium.
• Refraction occurs when light passes from one material into another and changes direction because its speed changes.

These processes explain how mirrors form images, how lenses focus light, and how the eye produces an image on the retina. They also underlie many medical instruments, including ophthalmoscopes, microscopes, endoscopes, and corrective lenses.