photoelectric effect

Let Us Learn About photoelectric effect

The photoelectric effect is usually observed when light is shined on metallic surfaces. The beam of light that is shone in a metal surface is referred to as the photocathode, and the electrons that it ejects from an atom are called photoelectrons. Shining light on a conductive metal surface can actually cause an electrical current, called a photocurrent , to form. A material that is sensitive to light, such as the metals which can carry an electrical current because of light, are referred to as photosensitive substances.

When light of sufficiently small wavelength is incident on a metal surface, electrons are ejected from the metal. This phenomenon is called as 'photoelectric effec

t' and the ejected electrons are called as 'photoelectrons

There are three ways in which electrons eject out of a material. They are

(i) Thermionic emission

(ii) Field emission

(iii) Photo electric emission

In all the above cases, energy is given to the material but in different forms. If given in the form of heat it is called as Thermionic emission, if in the form of electrical energy, it is field emission and if in the form of light (photons), then it is photoelectric effect.

The light must be energetic enough, which for zinc is in the ultraviolet region of the spectrum.If light were waves, we would expect the free electrons to steadily absorb energy until they escape from the surface. This would be the case in the classical theory, in which light is considered as waves. We could wait all day and still the red light would not liberate electrons from the zinc plate.So what is going on? We picture the light as quanta of radiation (photons). A single electron captures the energy of a single photon. The emission of an electron is instantaneous as long as the energy of each incoming quantum is big enough. If an individual photon has insufficient energy, the electron will not be able to escape from the metal.

There is a threshold frequency (i.e. energy), below which no electrons are released.

The electrons are released at a rate proportional to the intensity of the light (i.e. more photons per second means more electrons released per second).

The energy of the emitted electrons is independent of the intensity of the incident radiation. They have a maximum KE.

An analogy

Try this analogy, which involves ping-pong balls, a bullet and a coconut shy. A small boy tries to dislodge a coconut by throwing a ping-pong ball at it – no luck, the ping-pong ball has too little energy! He then tries a whole bowl of ping-pong balls but the coconut still stays put! Along comes a physicist with a pistol (and an understanding of the photoelectric effect), who fires one bullet at the coconut – it is instantaneously knocked off its support.

Ask how this is an analogy for the zinc plate experiment. (The analogy simulates the effect of infrared and ultra violet radiation on a metal surface. The ping-pong balls represent low energy infrared, while the bullet takes the place of high-energy ultra violet.)

Now you can define the work function. Use the potential well model to show an electron at the bottom of the well. It has to absorb the energy in one go to escape from the well and be liberated from the surface of the material.

Units

The electronvolt is introduced because it is a convenient small unit. You might need to point out that it can be used for any (small) amount of energy, and is not confined to situations involving electrically accelerated electrons.

Potential well

It is useful to compare the electron with a person in the bottom of a well with totally smooth sides. The person can only get out of the well by one jump, they can't jump half way up and then jump again. In the same way an electron at the bottom of a potential well must be given enough energy to escape in one 'jump'. It is this energy that is the work function for the material.

Now you can present the equation for photoelectric emission:

Energy of photon E = hf

Picture a photon being absorbed by one of the electrons which is least tightly bound in the metal. The energy of the photon does two things.

Some of it is needed to overcome the work function f.

The rest remains as KE of the electron.

hf = f + (1/2) mv^ ²

A voltage can be applied to bind the electrons more tightly to the metal. The stopping potential V_s is just enough to prevent any from escaping:

hf = f + eV_s