The Foundations of Chemistry

51.9

A photon of light near the middle of the ultraviolet region is more than 51 times more ener-
getic than a photon of light near the middle of the visible region.

You should now work Exercise 39.

THE PHOTOELECTRIC EFFECT

One experiment that had not been satisfactorily explained with the wave theory of light
was the photoelectric effect.The apparatus for the photoelectric effect is shown in Figure
5-13. The negative electrode in the evacuated tube is made of a pure metal such as cesium.
When light of a sufficiently high energy strikes the metal, electrons are knocked off its
surface. They then travel to the positive electrode and form a current flowing through
the circuit. The important observations follow.

1.Electrons can be ejected only if the light is of sufficiently short wavelength (has

sufficiently high energy), no matter how long or how brightly the light shines. This

wavelength limit is different for different metals.

2.The current (the number of electrons emitted per second) increases with increasing

brightness(intensity) of the light. The current, however, does not depend on the color

of the light, as long as the wavelength is short enough (has high enough energy).

Classical theory said that even “low-” energy light should cause current to flow if the
metal is irradiated long enough. Electrons should accumulate energy and be released when
they have enough energy to escape from the metal atoms. According to the old theory, if
the light is made more energetic, then the current should increase even though the light
intensity remains the same. Such is notthe case.
The answer to the puzzle was provided by Albert Einstein (1879–1955). In 1905, he
extended Planck’s idea that light behaves as though it were composed of photons,each with
a particular amount (a quantum) of energy. According to Einstein, each photon can transfer
its energy to a single electron during a collision. When we say that the intensity of light
is increased, we mean that the number of photons striking a given area per second is
increased. The picture is now one of a particle of light striking an electron near the surface
of the metal and giving up its energy to the electron. If that energy is equal to or greater
than the amount needed to liberate the electron, it can escape to join the photoelectric
current. For this explanation, Einstein received the 1921 Nobel Prize in physics.

5-11

1.81 10 
^17 J



3.49 10 
^19 J

Euv



Eyellow

This is one reason why ultraviolet

(UV) light damages your skin much

more rapidly than visible light.

Another reason is that many of the

organic compounds in the skin absorb

UV light more readily than visible

light. The absorbed ultraviolet light

breaks bonds in many biologically

important molecules.

5-11 The Photoelectric Effect 197

The intensity of light is the brightness

of the light. In wave terms, it is related

to the amplitude of the light waves.

Figure 5-13 The photoelectric

effect. When electromagnetic

radiation of sufficient minimum

energy strikes the surface of a metal

(negative electrode) inside an

evacuated tube, electrons are

stripped off the metal to create an

electric current. The current

increases with increasing radiation

intensity.

The photoelectric effect is used in the

photocells of automatic cameras. The

photoelectric sensors that open some

supermarket and elevator doors also utilize

this effect.

Light

Positive plate

Stream of

electrons

Active metal

(cathode)

Battery Ammeter