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CONCEPTS Applied Light

Polarization
1. Describe how you could test two pairs of polarizing – +
sunglasses to make sure they are polarized.
2. Look through the glass of a car windshield while
rotating a lens of a pair of polarizing sunglasses. What e
evidence do you find that the windshield is or is not
polarized? If it is polarized, can you determine the
direction of polarization and a reason for this?
3. Look at the sky through one lens of a pair of
polarizing sunglasses as you rotate the lens. What
evidence do you find that light from the sky is or is
not polarized? Is there any relationship between the
direction of polarization and the position of the Sun?
4. Position yourself with a wet street or puddle of water
between you and the Sun when it is low in the sky.
Rotate the lens of a pair of polarizing sunglasses as you
look at the glare reflected off the water. Explain how
these sunglasses are able to eliminate reflected glare.
FIGURE 7.23 A setup for observing the photoelectric effect.
Light strikes the negatively charged plate, and electrons are
ejected. The ejected electrons move to the positively charged plate
and can be measured as a current in the circuit.
magnetic fields changed the concept of light from mechanical

waves to waves of changing electric and magnetic fi elds. Further
evidence removed the necessity for ether, the material supposedly fewer electrons to be ejected, and high-intensity light caused
needed for waves to move through. Light was now seen as elec- many to be ejected, and (2) all electrons ejected from low- or
tromagnetic waves that could move through empty space. By high-intensity light ideally had the same kinetic energy. Surpris-
this time it was possible to explain all behaviors of light mov- ingly, the kinetic energy of the ejected electrons was found to be
ing through empty space or through matter with a wave theory. independent of the light intensity. This was contrary to what the

Yet there were nagging problems that the wave theory could wave theory of light would predict, since a stronger light should
not explain. In general, these problems concerned light that is mean that waves with more energy have more energy to give to
absorbed by or emitted from matter. the electrons. Here is a behavior involving light that the wave
theory could not explain.
PHOTOELECTRIC EFFECT
Light is a form of energy, and it gives its energy to matter when QUANTIZATION OF ENERGY
it is absorbed. Usually the energy of absorbed light results in a In addition to the problem of the photoelectric eff ect, there
temperature increase, such as the warmth you feel from absorbed were problems with blackbody radiation in the form of light
sunlight. Sometimes, however, the energy from absorbed light emitted from hot objects. Experimental measurements of this
has other effects. In some materials, the energy is acquired by light did not match predictions made from theory. In 1900,

electrons, and some of the electrons acquire suffi cient energy to Max Planck (pronounced “plonk”) (1858–1947), a German

jump out of the material. The movement of electrons as a result physicist, found that he could fit the experimental measure-
of energy acquired from light is known as the photo electric ments and theory together by assuming that the vibrating

eff ect. The photoelectric effect is put to a practical use in a molecules that emitted the light could only have a discrete

solar cell, which transforms the energy of light into an electric amount of energy. Instead of energy existing through a contin-
current (Figure 7.23). uous range of amounts, Planck found that the vibrating mol-
The energy of light can be measured with great accuracy. ecules could only have energy in multiples of certain amounts,

The kinetic energy of electrons after they absorb light can also be or quanta (meaning “discrete amounts”; quantum is singular,
measured with great accuracy. When these measurements were and quanta is plural).
made of the light and electrons involved in the photoelectric Planck’s discovery of quantized energy states was a radi-
effect, some unexpected results were observed. Monochromatic cal, revolutionary development, and most scientists, including

light, that is, light of a single, fixed frequency, was used to pro- Planck, did not believe it at the time. Planck, in fact, spent con-

duce the photoelectric effect. First, a low-intensity, or dim, light siderable time and effort trying to disprove his own discovery. It

was used, and the numbers and energy of the ejected electrons was, however, the beginning of the quantum theory, which was

were measured. Then a high-intensity light was used, and the eventually to revolutionize physics.
numbers and energy of the ejected electrons were again mea- Five years later, in 1905, Albert Einstein (1879–1955) applied
sured. Measurement showed that (1) low-intensity light caused Planck’s quantum concept to the problem of the photoelectric
7-17 CHAPTER 7 Light 193