Photoelectric effect?

Question

What is the practical example of Photoelectric effect?

Tell me some practical example of photoelectric effect.

Answer 1

Only point that I may add to nepthys_tom's excellent exposition and rhsaunder's very brief but pointed answer is that in the classical photoelectric effect electrons are emitted from a metal surface and travels across the photo cell as carriers of charge under the influence of an external e.m.f. A photo cell cannot act as a source of electricity but is useful as a detector and sensor of varying intensity of light.
On the other hand, a solid-state solar cell converts light energy into electrical energy and is used mainly as a source of electricity.

Answer 2

When light is incident on a metallic surface, electrons at the surface gain energy from the light. If this energy is greater than the work function of the metal (the energy required to free an electron from the surface), the electron will be ejected from the surface, acquiring some kinetic energy. From a wave picture of light, we would expect that the number of electrons ejected and their kinetic energy would increase when the light intensity is increased. We would also expect that the frequency of the light would not affect the kinetic energy of the ejected electrons. The particle or Einstein (he won the Nobel prize for explaining the photoelectric effect, after all) picture of light says that an increase in intensity ejects more electrons, but since the energy of each photon is unchanged, the kinetic energy of the ejected photons does not depend on the intensity. Furthermore, since the energy of a photon is h*f, where h is Planck's constant and f is the frequency of the light, the kinetic energy of an ejected electron is h*f - W, where W is the work function of the metal. Therefore, if h*f < W, no electrons will be ejected, regardless of the intensity. It turns out that Einstein was right. So, in summary, we have KE = h*f - W, which is increased when frequency is increased (this is equivalent to decreasing the wavelength of the light, but this is not an answer choice), so the answer is 2 only, or C. (Note that the number of photons incident on the surface does not matter. That is, only one photon may eject one electron. So, 3 is wrong.)

Answer 3

Solar Cells.

Answer 4

Solar cells. Photomultiplier tubes, used for detecting faint pulses of light.

Answer 5

Hi. Solar power.

Answer 6

Solar cells is one of the most common practical examples.

Answer 7

The Photoelectric Effect
What's the photoelectric effect?

It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. For example, you can start a current in a circuit just by shining a light on a metal plate. Why do you think this happens?

Well...we were saying earlier that light is made up of electromagnetic waves, and that the waves carry energy. So if a wave of light hit an electron in one of the atoms in the metal, it might transfer enough energy to knock the electron out of its atom.

Okay. Now, if light is indeed composed of waves, as you suggest... What do you mean, "if light is composed of waves"? Is there another option?

Historically, light has sometimes been viewed as a particle rather than a wave; Newton, for example, thought of light this way. The particle view was pretty much discredited with Young's double slit experiment, which made things look as though light had to be a wave. But in the early 20th century, some physicists--Einstein, for one--began to examine the particle view of light again. Einstein noted that careful experiments involving the photoelectric effect could show whether light consists of particles or waves.

How? It seems to me that the photoelectric effect would still occur no matter which view is correct. Either way, the light would carry energy, so it would be able to knock electrons around.

Yes, you're right--but the details of the photoelectric effect come out differently depending on whether light consists of particles or waves. If it's waves, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same. Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're using.

That sounds reasonable enough to me. How would the effect change if you assume that light is made of particles?

I should give you some background information on this, first. It all began with some work on radiation


Photoelectric theory

The photoelectric effect is important in history because it caused scientists to think about light and other forms of electromagnetic radiation in a different way. The peculiar thing about the photoelectric effect is the relationship between the intensity of the light shined on a piece of metal and the amount of electric current produced.
Words to Know

Anode: The electrode in an electrochemical cell at which electrons are given up to a reaction.

Cathode: The electrode in an electrochemical cell at which electrons are taken up from a reaction.

Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.

Electromagnetic radiation: Radiation (energy in the form of waves or subatomic particles) that transmits energy through the interaction of electricity and magnetism.

Frequency: The number of times a wave passes a given point in space per unit of time (as per second).

Photocell: A vacuum tube in which electric current flows when light strikes the photosensitive (or light sensitive) cathode.

Photon: A particle of light whose energy depends on its frequency.

Solar cell: A device constructed from specially prepared silicon that converts radiant energy (light) into electrical energy.

To scientists, it seemed reasonable that you could make a stronger current flow if you shined a brighter light on the metal. More (or brighter) light should produce more electric current—or so everyone thought. But that isn't the case. For example, shining a very weak red light and a very strong red light on a piece of metal produces the same results. What does make a difference, though, is the color of the light used.

One way that scientists express the color of light is by specifying its frequency. The frequency of light and other forms of electromagnetic radiation is the number of times per second that light (or radiation) waves pass a given point. What scientists discovered was that light of some frequencies can produce an electric current, while light of other frequencies cannot.

Einstein's explanation. This strange observation was explained in 1905 by German-born American physicist Albert Einstein (1879–1955). Einstein hypothesized that light travels in the form of tiny packets of energy, now called photons. The amount of energy in each photon is equal to the frequency of light (ν) multiplied by a constant known as Planck's constant (ℏ), or νℏ.

Einstein further suggested that electrons can be ejected from a material if they absorb exactly one photon of light, not a half photon, or a third photon, or some other fractional amount. Green light might not be effective in causing the photoelectric effect with some metals, Einstein said, because a photon of green light might not have exactly the right energy to eject an electron. But a photon of red light might have just the right amount of energy.

Einstein's explanation of the photoelectric effect was very important because it provided scientists with an alternative method of describing light. For centuries, researchers had thought of light as a form of energy that travels in waves. And that explanation works for many phenomena. But it does not work for phenomena such as the photoelectric effect and certain other properties of light.

Today, scientists have two different but complementary ways of describing light. In some cases, they say, it behaves like a wave. But in other cases, it behaves like a stream of particles—a stream of photons.
Applications

Two of the most important applications of the photoelectric effect are the photoelectric cell (or photocell) and solar cells. A photocell usually consists of a vacuum tube with two electrodes. A vacuum tube is a glass tube from which almost all of the air has been removed. The electrodes are two metal plates or wires. One electrode in a photocell consists of a metal (the cathode) that will emit electrons when exposed to light. The other electrode (the anode) is given a positive electric charge compared to the cathode. When light shines on the cathode, electrons are emitted and then attracted to the anode. An electron current flows in the tube from cathode to anode. The current can be used to turn on a motor, to open a door, or to ring a bell in an alarm system. The system can be made to respond to light, as described above, or it can be sensitive to the removal of light.

Photocells are commonly used in factories. Items on a conveyor belt pass between a beam of light and a photocell. As each item passes the beam, it interrupts the light, the current in the photocell stops, and a counter is turned on. With this method, the exact number of items leaving the factory can be counted. Photocells are also installed on light poles to turn street lights on and off at dusk and dawn. In addition, photocells are used as exposure meters in cameras. They measure the exact amount of light entering a camera, allowing a photographer to adjust the camera's lens to the correct setting.

Solar cells are devices for converting radiant energy (light) into electrical energy. They are usually made of specially prepared silicon that emits electrons when exposed to light. When a solar cell is exposed to sunlight, electrons emitted by silicon flow through external wires as a current.

Individual solar cells produce voltages of about 0.6 volts each. In most practical applications, higher voltages and large currents can be obtained by connecting many solar cells together. Electricity from solar cells is still quite expensive, but these cells remain very useful for providing small amounts of electricity in remote locations where other sources are not available. As the cost of producing solar cells is reduced, however, they will begin to be used for the production of large amounts of electricity for commercial use.