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A Closer Look

Lasers
he word laser is from light amplification tube with mirrors at both ends. The mir- Lasers are everywhere today and have
Tby stimulated emission of radiation. A ror at one end is only partly silvered, which connections with a wide variety of tech-
laser is a device that produces a coherent allows light to escape as the laser beam. The nologies. At the supermarket, a laser and
beam of single-frequency, in-phase light. distance between the mirrors matches the detector unit reads the bar code on each
The beam comes from atoms that have been resonant frequency of the light produced, grocery item. The laser sends the pattern
stimulated by electricity. Most ordinary light so the trapped light will set up an optical to a computer, which sends a price to the
sources produce incoherent light; light that standing wave. An electric discharge pro- register as well as tracks the store inven-
is emitted randomly and at different fre- duces fast electrons that raise the energy tory. A low-powered laser and detector also
quencies. The coherent light from a laser has level of the electrons of the specific gas read your CD music or MP3 disc and can
the same frequency, phase, and direction, so atoms in the tube. The electrons of the be used to make a three-dimensional image.
it does not tend to spread out and it can be energized gas atoms emit a particular Most laser printers use a laser, and a laser is
very intense. This has made possible a num- frequency of light as they drop back to a the operational part of a fiber optics com-
ber of specialized applications, and the list of lower level, and this emitted light sets up munication system. Stronger lasers are used
uses continues to grow. the standing wave. The standing wave for cutting, drilling, and welding. Lasers are
There are different kinds of lasers in stimulates other atoms of the gas, resulting used extensively in many different medical
use, and new ones are under development. in the emission of more light at the same procedures, from welding a detached retina
One common type of laser is a gas-filled frequency and phase. to bloodless surgery.

Both theories failed to explain some behaviors of light, such as of transverse waves vibrating in all conceivable random directions.
light moving through certain transparent crystals. For example, Polarizing materials, such as the tourmaline crystal, transmit light
a slice of the mineral tourmaline transmits what appears to be a that is vibrating in one direction only, such as the vertical direc-
low-intensity greenish light. But if a second slice of tourmaline tion in Figure 7.20B. Such a wave is said to be polarized, or
is placed on the first and rotated, the transmitted light passing plane-polarized since it vibrates only in one plane. Th e single

through both slices begins to dim. The transmitted light is prac- crystal polarized light by transmitting only waves that vibrate
tically zero when the second slice is rotated 90°. Newton sug- parallel to a certain direction while selectively absorbing waves
gested that this behavior had something to do with “sides” or that vibrate in all other directions. Your eyes cannot tell the dif-
“poles” and introduced the concept of what is now called the ference between unpolarized and polarized light, so the light
polarization of light. transmitted through a single crystal looks just like any other

The waves of Huygens’ wave theory were longitudinal, mov- light. When a second crystal is placed on the first, the amount

ing as sound waves do, with wave fronts moving in the direction of light transmitted depends on the alignment of the two crystals
of travel. A longitudinal wave could not explain the polarization (Figure 7.21). When the two crystals are aligned, the polarized

behavior of light. In 1817, Young modified Huygens’ theory by
describing the waves as transverse, vibrating at right angles to the
direction of travel. Th is modification helped explain the polar-

ization behavior of light transmitted through the two crystals
and provided firm evidence that light is a transverse wave. As

shown in Figure 7.20A, unpolarized light is assumed to consist

A
A

B
B
FIGURE 7.20 (A) Unpolarized light has transverse waves
vibrating in all possible directions perpendicular to the direction of FIGURE 7.21 (A) Two crystals that are aligned both transmit
travel. (B) Polarized light vibrates only in one plane. In this illustra- vertically polarized light that looks like any other light. (B) When
tion, the wave is vibrating in a vertical direction only. the crystals are crossed, no light is transmitted.

7-15 CHAPTER 7 Light 191