1.1 The propagation of light  (Page 3/6)

The ray model of light

You have already studied some of the wave characteristics of light in the previous chapter on Electromagnetic Waves . In this chapter, we start mainly with the ray characteristics. There are three ways in which light can travel from a source to another location ( [link] ). It can come directly from the source through empty space, such as from the Sun to Earth. Or light can travel through various media, such as air and glass, to the observer. Light can also arrive after being reflected, such as by a mirror. In all of these cases, we can model the path of light as a straight line called a ray    .

Experiments show that when light interacts with an object several times larger than its wavelength, it travels in straight lines and acts like a ray. Its wave characteristics are not pronounced in such situations. Since the wavelength of visible light is less than a micron (a thousandth of a millimeter), it acts like a ray in the many common situations in which it encounters objects larger than a micron. For example, when visible light encounters anything large enough that we can observe it with unaided eyes, such as a coin, it acts like a ray, with generally negligible wave characteristics.

In all of these cases, we can model the path of light as straight lines. Light may change direction when it encounters objects (such as a mirror) or in passing from one material to another (such as in passing from air to glass), but it then continues in a straight line or as a ray. The word “ray” comes from mathematics and here means a straight line that originates at some point. It is acceptable to visualize light rays as laser rays. The ray model of light describes the path of light as straight lines.

Since light moves in straight lines, changing directions when it interacts with materials, its path is described by geometry and simple trigonometry. This part of optics, where the ray aspect of light dominates, is therefore called geometric optics    . Two laws govern how light changes direction when it interacts with matter. These are the law of reflection , for situations in which light bounces off matter, and the law of refraction , for situations in which light passes through matter. We will examine more about each of these laws in upcoming sections of this chapter.

Summary

• The speed of light in a vacuum is $c=2.99792458\times {10}^8\text{m/s}\approx 3.00\times {10}^8\text{m/s}$ .
• The index of refraction of a material is $n=c\text{/}v,$ where v is the speed of light in a material and c is the speed of light in a vacuum.
• The ray model of light describes the path of light as straight lines. The part of optics dealing with the ray aspect of light is called geometric optics.
• Light can travel in three ways from a source to another location: (1) directly from the source through empty space; (2) through various media; and (3) after being reflected from a mirror.

Conceptual questions

Problems