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OVERVIEW

You use light and your eyes more than any other sense to learn about your surroundings. All of your other senses—
touch, taste, sound, and smell—involve matter, but the most information is provided by light. Yet light seems more
mysterious than matter. You can study matter directly, measuring its dimensions, taking it apart, and putting it
together to learn about it. Light, on the other hand, can only be studied indirectly in terms of how it behaves
(Figure 7.1). Once you understand its behavior, you know everything there is to know about light. Anything else is
thinking about what the behavior means.
The behavior of light has stimulated thinking, scientific investigations, and debate for hundreds of years. The
investigations and debate have occurred because light cannot be directly observed, which makes the exact nature of
light very difficult to pin down. For example, you know that light moves energy from one place to another place. You
can feel energy from the sun as sunlight warms you, and you know that light has carried this energy across millions
of miles of empty space. The ability of light to move energy like this could be explained (1) as energy transported by
waves, just as sound waves carry energy from a source, or (2) as the kinetic energy of a stream of moving particles,
which give up their energy when they strike a surface. The movement of energy from place to place could be explained
equally well by a wave model of light or by a particle model of light. When two possibilities exist like this in science,
experiments are designed and measurements are made to support one model and reject the other. Light, however,
presents a baffling dilemma. Some experiments provide evidence that light consists of waves and not a stream of
moving particles. Yet other experiments provide evidence of just the opposite, that light is a stream of particles and not
a wave. Evidence for accepting a wave or particle model seems to depend on which experiments are considered.
The purpose of using a model is to make new things understandable in terms of what is already known. When
these new things concern light, three models are useful in visualizing separate behaviors. Thus, the electromagnetic
wave model will be used to describe how light is created at a source. Another model, a model of light as a ray, a
small beam of light, will be used to discuss some common properties of light such as reflection and the refraction, or
bending, of light. Finally, properties of light that provide evidence for a particle model will be discussed before ending
with a discussion of the present understanding of light.

generates a water wave that spreads out from the rock, an accel-
7.1 SOURCES OF LIGHT
erating charge disturbs the electrical properties of the space
The Sun and other stars, lightbulbs, and burning materials all around it, producing a wave consisting of electric and magnetic

give off light. When something produces light, it is said to be fields (Figure 7.2). This wave continues moving through space

luminous. The Sun is a luminous object that provides almost all until it interacts with matter, giving up its energy.

of the natural light on Earth. A small amount of light does reach The frequency of an electromagnetic wave depends on
Earth from the stars but not really enough to see by on a moon- the acceleration of the charge; the greater the acceleration, the
less night. The Moon and planets shine by reflected light and do higher the frequency of the wave that is produced. Th e com-

not produce their own light, so they are not luminous. plete range of frequencies is called the electromagnetic spectrum

Burning has been used as a source of artifi cial light for (Figure 7.3). The spectrum ranges from radio waves at the low-

thousands of years. A wood fire and a candle flame are luminous frequency end to gamma rays at the high-frequency end. Visible
because of their high temperatures. When visible light is given light occupies only a small part of the middle portion of the

off as a result of high temperatures, the light source is said to be complete spectrum.

incandescent. A flame from any burning source, an ordinary Visible light is emitted from incandescent sources at high
lightbulb, and the Sun are all incandescent sources because of temperatures, but actually electromagnetic radiation is given off
high temperatures. from matter at any temperature. This radiation is called black-

How do incandescent objects produce light? One explana- body radiation, which refers to an idealized material (the black-
tion is given by the electromagnetic wave model. Th is model body) that perfectly absorbs and perfectly emits electromagnetic
describes a relationship between electricity, magnetism, and radiation. From the electromagnetic wave model, the radiation
light. The model pictures an electromagnetic wave as forming originates from the acceleration of charged particles near the

whenever an electric charge is accelerated by some external surface of an object. The frequency of the blackbody radiation
force. Just as a rock thrown into a pond disturbs the water and is determined by the temperature of the object. Near absolute
178 CHAPTER 7 Light 7-2