by Chris Woodford. Last updated: October 3, 2014.
Have you ever been in one of those restrooms where the faucets come on
automatically when you wave your hands underneath them? Or walked
through an electric door that opened just as you approached? Maybe
your home is fitted with invisible "magic-eye" beams that "trip
up" intruders by sounding an alarm? Or perhaps you've got a
calculator that makes power with a little built-in solar panel? All
these things are examples of photoelectric cells (sometimes
called photocells)—electronic devices that generate electricity when
light falls on them. What are they and how do they work? Let's take a
Photo: The mini solar panel on this pocket calculator uses a type
of photoelectric cell known as photovoltaic: when light falls on it, it produces enough voltage to power the display and the electronics inside.
What is photoelectricity?
"Photo" means light, so photoelectricity simply means electricity
produced by a light beam. That idea doesn't seem at all unusual in the 21st
century, when most people have heard of solar panels (lumps of
material, such as silicon, that generate an electric current when
sunlight shines on them). But imagine how amazing the photoelectric
effect must have seemed a little over a century ago, in 1887,
when it was first discovered by German physicist Heinrich Hertz
(1857–1894), one of the pioneers of radio. It remained something of a
mystery for almost 20 years until Albert Einstein weighed in with an
almost complete explanation of the phenomenon in 1905.
What is the photoelectric effect?
How can light magically transform itself into electricity? It's not as
strange as it sounds. We know, for example, that light is a kind of
electromagnetic energy: it travels in the same way (and at the same
speed) as X-rays,
radio waves, and other kinds of
electromagnetism. We also know that energy can readily be
transformed from one kind into another: potential energy can be
turned into kinetic energy and either can be converted into heat or
sound. So the idea that light might be turned into electricity isn't
all that surprising.
Yet, when photoelectricity was first explained, in 1905, it marked the
beginning of a scientific revolution. The person who did the
explaining, Albert Einstein (1879–1955), showed that a light beam, shining on
something like a piece of metal, could be thought of as a train of
energetic particles called photons. The photons passed their
energy in fixed quantities to atoms inside the metal, knocking some
of their electrons out of them, so producing an electric current.
As Einstein showed mathematically, the energy of the incoming photons was precisely
related to the frequency or wavelength of the light shining and equal
to the energy of the electrons they ejected. Einstein's explanation
of the photoelectric effect was powerful evidence that energy could
exist only in fixed amounts called quanta. (In other words, you can get energy in
family-sized packs but you can't split the packs up any smaller!)
This became the central
element of quantum theory: a complex, mathematical
explanation of the mysterious world of atoms and the particles
lurking inside them. And it was for this work on photoelectricity that Einstein won
the Nobel Prize in Physics 1921.
Photo: Albert Einstein won the Nobel Prize not for relativity—his best-known
contribution to physics—but for his earlier work on the photoelectric effect. Photo courtesy of
US Library of Congress.
Three types of photoelectricity
Photoelectricity is about light energy being converted into electrical energy and it
happens in three different (though, on the face of it, quite similar)
ways. They're known as the photoconductive, photoemissive, and
photovoltaic effects—and we'll look at each one in turn.
Incidentally, when I talk about light in this article, I don't just mean the
"visible" light we can see: photoelectric cells also work with
invisible forms of light such as infrared and ultraviolet:
light-sensitive materials can "see" and respond to frequencies of
light outside the range to which our own eyes are sensitive.
This is the easiest of the three effects to understand. When I was a
teenager, I remember briefly playing around with an electronic
component called a light-dependent resistor (LDR). It was like a
small button with two terminals coming out of the back and you could
solder it into a circuit much like any other
resistor. The surface of
the "button" had a lens on top of it
(to concentrate incoming light) and, under the lens, there
was a piece of light-sensitive material made from something like
calcium sulfide, with a snake-like pattern of electrical connections running across
it. In darkness or normal light, the LDR had a fairly high resistance
but if you shone a light directly at it, the resistance decreased
quite dramatically: the LDR was converting incoming light into
electrical energy and adding it to the current already passing
through. This is an example of the photoconductive effect, where
light reduces the resistance of a material (or increases its
conductance, if you prefer) by making the electrons inside it more
Photo: A typical light-dependent resistor (LDR).
Small solar panels on such things as calculators and
digital watches are
sometimes referred to as photovoltaic cells. They're a bit like
diodes, made from two layers of semiconductor material placed on top
of one another. The top layer is electron rich, the bottom layer,
electron poor. When you shine light on the top layer, electrons leap
up from the bottom layer to the top, making a voltage that can drive
current through an external circuit—so providing what we think of as
solar power. Read more about photovoltaics in our main article on
Photo: A solar panel made from photovoltaic cells.
Photo by Warren Gretz courtesy of US DoE/NREL.
Photoemissive cells are are the oldest and most elaborate way of turning light into
electricity. They're sealed glass vacuum tubes (from which the air
has been completely removed), inside which there's a large metal
plate that serves as a negative terminal (or cathode) with a smaller,
positively charged, rod-like terminal (or anode) running inside it.
The negative terminal is made from a light-sensitive material. When
light photons fall on it, they force electrons to leap out of it and
these are promptly attracted to the positive terminal, which collects
them and channels them into a circuit, producing electric power. This
basic design is called a photoemissive cell or phototube.
In a slightly different design called a photomultiplier,
there's a whole series of plates arranged so that one
incoming photon releases multiple electrons—effectively amplifying
an incoming light signal so it produces a bigger electrical response.
Photo: Right: A basic phototube.
Artwork: A summary of the three types of photoelectric cells.
Left: Photoconductive—light increases the flow of electrons and reduces the resistance.
Middle: Photovoltaic—light makes electrons move between layers, producing a voltage and a current in an external circuit.
Right: Photoemissive—light knocks electrons from a cathode to an anode, making a current flow through an external circuit.
What are photoelectric cells used for?
All three types of photoelectric cell can detect light or convert it into
electricity, but in practice they have quite different uses.
Like miniature power plants,
photovoltaic cells are designed to produce
steady supplies of useful, electric power. From small solar cells on
electronic calculators to completely photovoltaic roofs, their job is
essentially to produce a constant supply of electricity that we can
use to power electric appliances or store in batteries for later.
Photo: Left: A photoelectric security light: When the photoelectric detector (bottom) senses movement,
the light (top) switches on automatically for several minutes.
Photoconductive cells such as light-dependent resistors are more likely to be used as
light detectors in such things as automated washroom taps, intruder
alarms, doorways that open automatically, smoke alarms,
carbon monoxide detectors, and so on. Typically, they have a beam of
infrared light shining permanently on a light-dependent resistor and
producing a steady electric current. When you move in front of the
detector, you break the beam and stop the light reaching the resistor, so its resistance
changes and it suddenly produces much less current. An electronic circuit detects
the change in current and triggers whatever action the circuit is
designed to take—turning on a faucet, opening a door, sounding an
alarm, or whatever it might be. Photoconductive cells are also used
as light detectors in cameras and for reading and decoding the
soundtracks on old-style movie reels.
Photo: Right: How can you tell male flies from female ones? Melon fly pupae are
either brown (if they're male) or white (if they're female). They can be separated by tipping them into a photoelectric
sorter, which shines a light on each pupa, detects how much light is reflected back with a photocell, and then sifts
the pupa into one box or the other according to its color. The same apparatus can be used for sorting seeds.
Photo Stephen Ausmus courtesy of US Department of Agriculture Agricultural Research Service.
Phototubes were originally used as light detectors too, but they're relatively
cumbersome, elaborate, and expensive; smaller and cheaper electronic
components like LDRs are now more widely used as light-detectors instead.
Photomultipliers are still used in scientific applications, such as
detecting radiation of different kinds, and in gadgets like
night vision goggles, where they intensify the dim light of a night-time
scene so it can be seen more clearly.
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