FIGURE 34.2 illustrates the idea that objects—sources of light—can be either self-luminous, such as the sun, flames, and lightbulbs, or reflective. Most objects are reflective. A tree, unless it is on fire, is seen or photographed by virtue of reflected sunlight or reflected skylight. People, houses, and pages in a printed book reflect light from self-luminous sources. In this chapter we are concerned not with how the light originates but with how it behaves after leaving the object.

FIGURE 34.2 Self-luminous and reflective objects.

Light rays from an object are emitted in all directions, but you are not aware of light rays unless they enter the pupil of your eye. Consequently, most light rays go completely unnoticed. For example, light rays travel from the sun to the tree in Figure 34.2, but you’re not aware of these unless the tree reflects some of them into your eye. Or consider a laser beam. You’ve probably noticed that it’s almost impossible to see the beam from a laser pointer as it crosses the room unless there’s dust in the air. The dust scatters a few of the light rays toward your eye, but in the absence of dust you would be completely unaware of a very powerful light beam traveling past you. Light rays exist independently of whether you are seeing them.

FIGURE 34.3 shows two idealized sets of light rays. The diverging rays from a point source are emitted in all directions. It is useful to think of each point on an object as a point source of light rays. A parallel bundle of rays could be a laser beam. Alternatively it could represent a distant object, an object such as a star so far away that the rays arriving at the observer are essentially parallel to each other.

FIGURE 34.3 Point sources and parallel bundles represent idealized objects.