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Visible light Incident
light
Sensitivity of
human eye
Relative intensity radiation radiation Absorbed
Ultraviolet
Infrared

light
Transmitted
light
FIGURE 7.6 Light that interacts with matter can be reflected,
absorbed, or transmitted through transparent materials. Any
combination of these interactions can take place, but a particular
7.5 x 10 14 3.8 x 10 14 substance is usually characterized by what it mostly does to light.
Frequency (Hz)
always been used to suggest that “something” travels in straight-
FIGURE 7.5 Sunlight is about 9 percent ultraviolet radiation,
line paths. The light ray is a line that is drawn to represent the

40 percent visible light, and 51 percent infrared radiation before it
travels through Earth’s atmosphere. straight-line travel of light. A line is drawn to represent this

imaginary beam to illustrate the law of reflection (as from a mir-

ror) and the law of refraction (as through a lens). There are limits
The incandescent flame of a candle or fire results from the

blackbody radiation of carbon particles in the flame. At a black- to using a light ray for explaining some properties of light, but it
body temperature of 1,500°C (about 2,700°F), the carbon parti- works very well in explaining mirrors, prisms, and lenses.
cles emit visible light in the red to yellow frequency range. Th e

tungsten filament of an incandescent lightbulb is heated to about LIGHT INTERACTS WITH MATTER
2,200°C (about 4,000°F) by an electric current. At this tempera-
ture, the visible light emitted is in the reddish, yellow-white range. A ray of light travels in a straight line from a source until it

The radiation from the Sun, or sunlight, comes from the encounters some object or particles of matter (Figure 7.6). What
Sun’s surface, which has a temperature of about 5,700°C (about happens next depends on several factors, including (1) the
10,000°F). As shown in Figure 7.5, the Sun’s radiation has a broad smoothness of the surface, (2) the nature of the material, and
spectrum centered near the yellow-green frequency. Your eye (3) the angle at which the light ray strikes the surface.

is most sensitive to this frequency of sunlight. The spectrum of Th e smoothness of the surface of an object can range from
sunlight before it travels through Earth’s atmosphere is infrared perfectly smooth to extremely rough. If the surface is perfectly
(about 51 percent), visible light (about 40 percent), and ultra- smooth, rays of light undergo refl ection, leaving the surface par-
violet (about 9 percent). Sunlight originated as energy released allel to one another. A mirror is a good example of a very smooth

from nuclear reactions in the Sun’s core (see p. 355). Th is energy surface that reflects light in this way (Figure 7.7A). If a surface is
requires about a million years to work its way up to the surface. not smooth, the light rays are refl ected in many random direc-
At the surface, the energy from the core accelerates charged parti- tions as diff use refl ection takes place (Figure 7.7B). Rough and
cles, which then emit light. The sunlight requires about 8 minutes irregular surfaces and dust in the air make diff use refl ections.

to travel the distance from the Sun’s surface to Earth. It is diff use refl ection that provides light in places not in direct
lighting, such as under a table or under a tree. Such shaded areas

would be very dark without the diff use reflection of light.
Some materials allow much of the light that falls on them
7.2 PROPERTIES OF LIGHT
to move through the material without being refl ected. Mate-
You can see luminous objects from the light they emit, and you rials that allow transmission of light through them are called
can see nonluminous objects from the light they reflect, but you transparent. Glass and clear water are examples of transparent

cannot see the path of the light itself. For example, you cannot materials. Many materials do not allow transmission of any light

see a flashlight beam unless you fill the air with chalk dust or and are called opaque. Opaque materials reflect light, absorb

smoke. The dust or smoke particles reflect light, revealing the light, or do some combination of partly absorbing and partly

path of the beam. This simple observation must be unknown reflecting light (Figure 7.8). The light that is refl ected varies with

to the makers of science fiction movies, since they always show wavelength and gives rise to the perception of color, which will
visible laser beams zapping through the vacuum of space. be discussed shortly. Absorbed light gives up its energy to the
Some way to represent the invisible travels of light is needed material and may be reemitted at a different wavelength, or it

in order to discuss some of its properties. Th roughout history, may simply show up as a temperature increase.
a light ray model has been used to describe the travels of light. Th e angle of the light ray to the surface and the nature of the

The meaning of this model has changed over time, but it has material determine if the light is absorbed, transmitted through
180 CHAPTER 7 Light 7-4