Download Visible light stimulates sensing cells in our eyes. We sense the

You learned in the last chapter
that light is electromagnetic
radiation. But the word “light” is
a broad term that includes three
regions of the electromagnetic
spectrum. Light includes
infrared radiation, visible
radiation, and ultraviolet
radiation. Radiation from the sun
contains all of these wavelengths
of light.
The wavelengths of infrared
radiation are too long for our
eyes to sense. We feel infrared
radiation as heat on our skin.
Visible light stimulates sensing cells in our eyes. We sense the longer
wavelengths of visible light as red light and the shorter as blue light.
The wavelengths of ultraviolet radiation are too short for our eyes to sense.
This high-energy radiation produces both tanning and burning of our skin.
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Figure 10.1
The electromagnetic spectrum
From Plato’s time in ancient Greece to Max Planck in the 1920s,
philosophers and scientists argued about whether light was a particle or a
wave. Their confusion can be understood because electromagnetic radiation
has both characteristics. Sometimes light acts like a wave as it passes
through lenses in telescopes or our eyes. Sometimes light acts like tiny
particles that are bundles of energy called photons. When a photon hits a
grain of photographic emulsion on film, the photon energy supplies the
energy to cause a chemical reaction. An electron in a neon atom in a laser
can lose energy and produce a photon. Each of these ways of thinking about
light can be helpful in different situations.
Figure 10.2
Light is represented by rays.
When we want to represent the path that light follows, we draw an arrow or
ray. In Figure 10-2, light is shown moving from a lamp, down onto a
tabletop, along straight lines. Then the light reflects off the tabletop and
moves away along other straight lines. The direction along which the light
travels is shown by the head of the arrow. Light rays drawn this way
represent beams of light that are composed of millions of light photons
traveling together.
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From your study of Chapter 8, you know that water and sound waves
transfer energy. Light waves also carry energy from a source, such as the sun
or a light bulb, to other places. Like water waves that travel outward along
the water surface when a rock is dropped into a pond, light waves from the
sun travel outward through space. In this chapter you’ll learn about the
behavior and control of light by dealing first with light rays and then with
light waves.
(a) Water waves move energy outward.
(b) Light waves move energy outward.
Figure 10.3
Water waves and light waves transfer energy.
Optical systems are made up of light sources, a variety of optical
components, detectors, and computers that use light to perform useful
functions. Our eyes are very complex optical systems that use light to form
images. These images are interpreted by our brains.
Combinations of laser light sources, telescopes, light detectors, and
microcomputers make up optical surveying systems that locate roads,
bridges, and buildings with millimeter precision. Diode lasers and
photodiode detectors are components in optical systems that record and read
CDs. The Hubble telescope uses mirrors and lenses to form images. Each
image is divided into pixels—a small subdivision of the image—which can
be described by numbers and transmitted to Earth. Computers convert the
numbers back into a visible image. All of the components needed to
complete this process form an optical system.
Figure 10.4
Hubble telescope
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One of the most useful optical systems in science and technology is the
laser. Energy can be “pumped” into the electrons of certain atoms or
molecules and stored there for a short time. When that energy is released
from the electrons in the form of photons, all the photons emitted have the
same energy and wavelength. This stream of photons makes a beam of light
that is very pure in color (they have very nearly the same wavelength). All
the wave components of the beam are phase coherent, that is, they are in
step as shown in Figure 10.5.
Figure 10.5
Coherent waves are in phase with each other.
In addition to the pure color and phase coherence, the physical construction
of most lasers produces a light beam the spreads very little over long
distances. This characteristic can be improved with some external lenses.
For example, a lens enables a beam sent from the Earth to the moon to have
a diameter of only few hundred meters when it hits the moon.
These three properties of laser light—color purity, coherence, and low
divergence—allow laser light to be focused to very small spots and produce
very high power densities. High power densities are used for laser surgery,
metal cutting, and trimming electronic parts.
(a) Cutting metal
(b) Alignment
(c) Eye surgery
(d) Surveying
Figure 10.6
Uses of laser light
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