What the scientists observed does not make sense if light is conceived of solely as a wave. Let’s compare their observations in terms of water

waves crashing against a wooden dock. It is as if low-frequency waves (with their crests arriving, say, every five seconds) could never rattle

the dock enough to knock free the timbers that make it up, even if they were giant waves 50 meters tall.

Now imagine centimeter-high waves arriving more frequently, say every second. Imagine that these small but frequent waves could knock

loose pieces of the wood from the dock. Water waves with these effects would be confusing to observe, and you might be as confused as the

scientists who observed dim but high frequency light freeing electrons from samples of metal.

Einstein successfully explained the photoelectric effect. He argued that light has both a wave nature and a particle nature. Electromagnetic

radiation, he said, consists of small packets called photons. Photons are small “chunks” of light energy. More energetic light consists of more

photons, not larger, more energetic waves. In addition, he stated that the higher the frequency of light, the more energetic the photons that

make up the light. Dim but high frequency light can eject electrons because of the interaction between the energetic photons that make it up

and the atoms of the metal. It is the energy of the individual photons that matters, not the overall energy of the light.

Conceiving of light as consisting of photons could explain another mysterious result: More intense light of a certain color caused more electrons

to be emitted, but their maximum kinetic energy was exactly the same. The classical physicist would expect the “larger wave” of the more

intense light to cause higher-energy electrons to be released, but this did not happen. Einstein’s theory explains why: more intense light

consists of more photons, each with the same energy as before. Again, it is the interaction between an individual photon and an individual atom

that matters.

You just read a brief summary of some crucial points in quantum physics. You will become familiar with the photoelectric effect by using the

simulation on the right. In your experiment with this effect, the flashlight can shine red or violet light. It can be set to emit either low or high

intensity light.

When you press GO, you will see photons moving in slow motion from the flashlight toward the metal. When appropriate, we show electrons

escaping from the metal.

Start the simulation with the light set to LOW. One color of light will cause electrons to escape the metal being used in our simulation; another

will not. Red light has a longer wavelength but a lower frequency than violet light. Which of the two colors of light do you think will cause

electrons to be emitted?

Now set the intensity of the light to HIGH, and try both colors again. What do you expect will change when you make this change? What do you

think will stay the same?

36.1 - Quantum

Quantum: The smallest amount of something

that can exist independently.

Quantum refers to the fundamental or least amount of something. For instance, the

quantum of money in the U.S. is the penny. Your net worth will be a multiple of that

quantum. You can be a pauper worth one penny, a millionaire with a worth of

100,000,000 pennies, or a starving college student with a net worth of í5,012 pennies.

However, you cannot legally use three-quarters of a penny, or 1.45 pennies, or 3 ʌ/4

pennies. There are many similar examples of things that come in discrete amounts: the

number of siblings you have, the number of eggs you can purchase at a store, and so

on.

A physicist would say that things like money or siblings or eggs are quantized. Calling

something quantized means that it is grainy; it is the opposite of continuous. Using the

example mentioned in this chapter’s introduction, one would say that the height and

energy of a bucket being raised by a rope are continuous quantities, as are the height

and energy of an elevator car. In contrast, the height and energy of elevator stops are

quantized; they occur solely at discrete points. You only see buttons for the first, second

and third floors, not for the 1.75th floor.

A mathematical example of something that is continuous is shown on the right: real

numbers. Examples of real numbers are 3, or 3.1, 3.01, 3.001, 3.002 and so forth. The

set of real numbers is continuous, not quantized.

Although the idea of quantization may seem intuitive for money, it is much less obvious

in some areas of physics: Scientists now know that many things once thought to be

continuous are in fact quantized. Albert Einstein, for instance, showed that the energy of

any precise color of light is quantized.

Prior to Einstein scientists expected properties of light, such as its energy, to be

continuous. Why? In the 18th and 19th centuries, a series of discoveries had led most

scientists to conclude that light was a wave. They knew very well that the energy of a

mechanical wave is continuous, not quantized. An ocean wave, for instance, can have a

height (amplitude) of 1.01 meters, or 1.04 meters, or any value in between, and its energy will depend on that amplitude. Since light was

believed to be a wave, scientists concluded that its energy would be continuous as well, and that for example, they could create a beam of a

certain frequency of blue light with any desired energy simply by making the light brighter or darker.

However, as the next sections discuss, in the early 20th century it became increasingly clear that light is quantized: It consists of small chunks

Quantized things

Exist with discrete values

Opposite of quantized

Continuous

·Real numbers are an example

(^660) Copyright 2007 Kinetic Books Co. Chapter 36