The Photoelectric Effect

Photoelectric Effect

Certain metals emit electrons from their surface when light shines on them. Active metals, such as potassium and calcium, emit electrons when they are exposed to visible light. Many other metals emit electrons tinder the action of ultraviolet light. Emission of electrons, by shining light on certain metal surfaces, is called’ the photoelectric effect (Fig.) and the emitted electrons are known as photoelectrons.

There are three significant features of the photoelectric effect:

The number of electrons knocked out of the metal surface depends on the intensity (brightness) of the incident light; the . energies of the ejected electrons do not,

The electron energies depend on the frequency. (colour) rather than the intensity of light. The higher the frequency of light; the greater the energies of the ejected electrons. Thus, the freed electrons have greater energies when a photoelectric material is struck by a feeble blue light than by a bright red one.

With diminishing frequency of incident light, the energies of the ejected electrons decrease. At a, particular frequency for each metal (called its threshold frequency). the photoelectric effect vanishes even under a very high intensity of light.

That is each metal requires a characteristic minimum frequency ^–before any electrons are ejected. For example, red light U 4.3 4.6 x 10¹⁴Hz) of intensity could shine_. on potassium all day without causing the photoelectric effect.

However, when a very weak yellow light U 5.1— 5.2x 10¹⁴ Hz) shines on potassium the photoelectric effect is immediately produced.

The wave theory of light could not explain these features of photoelectric effect  According to the wave theory, the number of ejected electrons as well as their energies would depend on the intensity of the incident light and not upon its wavelength.

Einstein used the quantum theory to explain the photoelectric effect. He proposed that the light striking the metal surface is a stream of indivisible particles, called photons.

When a photon hits an electron of the photoelectric metal, it gives up its entire energy to the electron because the photon cannot decompose into parts. A certain amount of energy is required for the electron to. overcome the attractive forces that hold it within the metal. If the energy were less; it would not escape from the metal surface, howsoever high the intensity of light may be.

That is, a photon must have a certain minimum energy to be able to dislodge the electron. This Minimum energy is given by E = hu_o, where u_o is the threshold frequency.

If a photon has more energy (hu) than the minimum energy ( hu_o ) required td free an electron, the difference is imparted to the ejected electron as kinetic energy 1/2 mv² , where m is the mass of the electron and v its velocity. That is, I hu- hu_o = 1/2 mv²

Since for any particular metal u_o and h are constants, the kinetic energy of the ejected electron is proportional to the frequency, u , of the radiation. Any increase in the intensity of light of a given frequency increases the number of photons but does not increase their energy. That is, when the intensity of light is increased, more electrons are ejected, but the energies of ejected electrons do not change.