1. 2. 3. The assumption that light travels in a straight line is the basis of the Ray model of light The straight-line path traversed by light is called a ray. A light ray approaching the surface is incident ray while the light leaving the surface is called reflected ray. The normal line is the imaginary line drawn from the point of incidence perpendicular to the surface that bisects the diagram into two equal parts. The angle an incident ray makes with the normal is called the angle of incidence. The angle the reflected ray makes with the normal is called the angle of reflection The Law of Reflection states that the angle of incidence is equal to the angle of the reflected ray The mathematical representation of the law of reflection is er = = 0 5. 6. 7. 8. Diffuse reflection is when light is incident upon a rough surface and is reflected in many directions creating a hazy and not so clear image of the object. 9. Specular reflection occurs when light is incident upon a smooth surface and angle is reflected on the same angle creating a clear and precise image of the object 10. Mirrors are good example of reflecting devices. 11. The image is formed because of reflection.

There are three ways in which light can travel from a source to another location. (See Figure 1.) 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 person. Light can also arrive after being reflected, such as by a mirror. In all of these cases, light is modeled as traveling in straight lines called rays. 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 (or even science fiction depictions of ray guns).

RAY

The word “ray” comes from mathematics and here means a straight line that originates at some point.

Part a shows the suns rays shining through the atmosphere to a house on the earth. The second image shows a woman looking out the window at a car and arrows depicting the light rays hitting the car and the window, then her eyes are shown.

Figure 1. Three methods for light to travel from a source to another location. (a) Light reaches the upper atmosphere of Earth traveling through empty space directly from the source. (b) Light can reach a person in one of two ways. It can travel through media like air and glass. It can also reflect from an object like a mirror. In the situations shown here, light interacts with objects large enough that it travels in straight lines, like a ray.

Experiments, as well as our own experiences, show that when light interacts with objects several times as large as 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 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 light encounters anything we can observe with unaided eyes, such as a mirror, it acts like a ray, with only subtle wave characteristics. We will concentrate on the ray characteristics in this chapter.

Since light moves in straight lines, changing directions when it interacts with materials, it is described by geometry and simple trigonometry. This part of optics, where the ray aspect of light dominates, is therefore called geometric optics. There are two laws that 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.

GEOMETRIC OPTICS

The part of optics dealing with the ray aspect of light is called geometric optics.

Section Summary

A straight line that originates at some point is called a ray.

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; (3) after being reflected from a mirror.

PROBLEMS & EXERCISES

Suppose a man stands in front of a mirror as shown in Figure 2. His eyes are 1.65 m above the floor, and the top of his head is 0.13 m higher. Find the height above the floor of the top and bottom of the smallest mirror in which he can see both the top of his head and his feet. How is this distance related to the man’s height?

A full-length mirror is one in which you can see all of yourself. It need not be as big as you, and its size is independent of your distance from it.

Figure 2. A man standing in front of a mirror on a wall at a distance of several feet. The mirror’s top is at eye level, but its bottom is only waist high. Arrows illustrate how the man can see his reflection from head to toe in the mirror.

Glossary

ray: straight line that originates at some point

geometric optics: part of optics dealing with the ray aspect of light

Explanation: