Photoelectric Effect

Introduction

During his electromagnetic wave experiments in 1887, Heinrich Hertz discovered the phenomenon of the photoelectric effect. He observed, the escape of free-charged particles, which we now know as electrons, was somehow facilitated by the light shining on the metal surface. When the light fell on the metal surface, a few electrons near the surface of the metal absorbed enough energy from the incident radiation to overcome the forces of attraction. Once they gain sufficient power from the incident light, the electron escapes from the surface of the metal into the surrounding space.

When the light of a suitable frequency illuminates a metal surface, electrons are emitted from the metal surface, known as the photoelectric effect.  This effect has been found helpful in electronic devices specialized for detecting light and precisely timed emissions of electrons.

Principle of the Photoelectric Effect

During 1862 – 1902 Wilhelm Hallwachs and Philip Lenard investigated the phenomenon of the photoelectric effect in detail. Lenard’s study showed that current started flowing in the circuit when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes. As soon as they were stopped, the current flow was also stopped. These observations led him to stipulate that ultraviolet radiations falling on the emitter plate ejected electrons which then were attracted towards the positive collector by an electric field. Thus, light falling on the surface of the emitter plate caused electric current in the external circuit.

• Threshold Frequency

Hallwachs and Lenard also observed that when the frequency of the incident UV light was smaller than a specified minimum value, known as the threshold frequency, then no electrons were emitted at all. This minimum frequency was dependent on the nature of the material of the emitter plate. Metals like zinc, cadmium, magnesium, etc., were found to respond only to UV light with short wavelengths to cause electron emission from the surface. However, other alkali metals such as lithium, sodium, potassium, cesium, and rubidium were sensitive even to visible light. The current produced due to the photoelectric effect is known as photoelectric current.

• Properties of Photoelectric Current

The experimental study of the photoelectric effect gave us the following observations.

• Effect of Intensity of Light on Photocurrent

The photoelectric current is directly proportional to the intensity of the incident light for a given photosensitive material and the frequency of incident radiation above the threshold frequency.

• Effect of Potential on Photocurrent

The stopping potential is independent of light intensity. In contrast, the saturation current is proportional to the power of incident radiation for a given photosensitive material and the frequency of incident radiation.

• Effect of Frequency of Incident Radiation on Stopping Potential

The stopping potential or the maximum kinetic energy of the emitted photoelectron increases linearly with the frequency of incident radiation. Still, it is independent of its intensity above the threshold frequency for a given photosensitive material.

K_Max = eV_o

• Einstein’s Photoelectric Equation

Albert Einstein, in 1905 proposed that radiation energy is built up of discrete units called quanta of energy radiation. Each quantum of radiation has energy h𝝂, where h is Planck’s constant and 𝝂 is the frequency of light radiation. The electron absorbs a quantum of energy. If it exceeds the minimum energy needed for the electron (Work Function ɸ0)  to escape from the metal surface, the electron is emitted with the maximum kinetic energy.

K_max= h-0 = h-h₀

The photoelectric process is instantaneous regardless of the light intensity. Since the basic elementary operations, low power does not mean a delay in emission. Only the number of electrons that can participate in the absorption of light quanta by a single electron is determined by the intensity of the light radiation and, therefore, the photoelectric current.

eV₀ = h-0 for >0

V0= (he)-0e

The “VO vs v” is a straight line with slope = he

Particle Nature of Light.

The evidence given to us by the photoelectric effect led to a strange fact that light in interaction with matter behaved as if it was made of quanta or packets. Einstein discovered an important result that the light quanta could also be associated with momentum, where c is the speed of light. The light quanta, therefore, was associated with a particle, as a definite value of energy and momentum is a strong sign of its particle nature. This particle was later named the Photon. The experiment of scattering X-rays from electrons, performed by H. Compton in 1924, confirmed the behavior of light as a particle.

The Photon

1. When radiation interacts with matter, it behaves as if it is made up of photons.
2. The energy (E) and momentum (p) of each photon are and E = h and p = h/crespectively.
3. The energy and momentum of all photons of light of a particular frequency or wavelength () are the same regardless of radiation intensity. By increasing the intensity of light radiation for a given wavelength, only an increase in the number of photons per second crossing a given area takes place, with each photon having the same energy. Thus, the energy of each photon is independent of the intensity of light radiation.
4. Electric and magnetic fields do not deflect photons as they are electrically neutral.
5. The total energy and momentum are conserved in a photon-particle collision (such as a photon-electron collision). The number of photons may not be conserved in the collision, and this may lead to the number of photons being absorbed or a new photon being created.

Conclusion

In this article, we learned about the photoelectric effect that was theoretically explained by Albert Einstein, for which he was awarded the Nobel Prize in Physics in 1921, and its principles. When the light of a suitable frequency illuminates a metal surface, electrons are emitted from the metal surface and is known as the photoelectric effect. We also learned about the threshold frequency, the work function of the photoelectric current, and its properties. The minimum frequency required for electron emission is known as the threshold frequency. We also learned about the particle nature of light and about the photon. The light quantum is associated with a particle and has a definite value of energy as well as momentum and is known as the Photon.  This chapter is very important, and to gain a better understanding of it, other topics such as the wave nature of light and De Broglie’s equation are suggested to the user.