Laws of Photoelectric Effect

The emission of electrons when ultraviolet rays or light rays are incident on the material is known as the photoelectric effect. The emitted electrons are known as photoelectrons. The process of emission of electrons is known as Photoelectric emission. Over the years, scientists have studied the process and come to certain conclusions known as the “Laws of Photoelectric Effect”. 

The photoelectric effect has helped develop various technologies. For example, it has played an essential role in developing solar panels to harness solar energy. In addition, the photoelectric effect laws help us understand photoelectric emission. The phenomenon has been significant in the development of modern physics and has found huge applications.

History

In 1887, the German physicist Heinrich Rudolf Hertz discovered the photoelectric effect. During his work on radio waves, Hertz observed when ultraviolet light shines on two charged metal electrodes, the light changes the voltage at which sparking occurs. 

In 1902, German physicist Philipp Lenard further clarified light and electricity. He observed that a metal surface on illumination liberates electrically charged particles. These particles were discovered in 1897 by British physicist Joseph John. According to him, the particles stand identical to electrons.

Many scientists experimentally observed the effect. Some observations showed an interaction between light and matter that classical physics cannot explain. 

Another unexplainable observation was the relation between the maximum kinetic energy of the released electrons and the intensity of the light. According to the wave theory, the kinetic energy was proportional instead to the frequency of the incident light.

After conducting different experiments, the scientists outlined specific experimental observations. They then combined the observations and listed the Laws of the Photoelectric Effect.

Experimental Observations

1. The threshold frequency is a specific minimum frequency above which the photoelectric effect occurs for a given metal.
2. By increasing the frequency of incident light, there is an increase in the maximum kinetic energy of the emitted photoelectrons. However, the number of incident photons remains fixed.
3. Once the photoelectrons surpass the threshold frequency, their maximum kinetic energy depends only on the frequency of incident light. The intensity of incident light has no control over the kinetic energy of the photoelectrons. 
4. The rate of photoelectrons’ emission is directly proportional to the intensity of incident light for a given metal and a particular frequency of light. Thus by increasing the magnitude of light, the photoelectric current value increases.
5. The time lag between photons’ incidence and photoelectric effect’s radiation is minimal, nearly 10-9 seconds.

Laws of Photoelectric Effect

Laws of the photoelectric effect are as follows:

1. A substance does not emit electrons beyond the cut-off value frequency.
2. The number of emitted electrons is directly proportional to the intensity of the light incident.
3. The kinetic energy of emitted electrons depends on the frequency of incident light on the substance.
4. There is no time lag observed between the light incident and the emission of electrons.

Properties of the Photon

To understand the photoelectric effect, one also needs to know photon properties. These are: 

• The quantum numbers are zero for photons.
• A photon does not have mass.
• The magnetic and electric field does not affect a photon.
• The speed of a photon is directly proportional to the speed of light in space.
• When a matter interacts with radiation, the radiation behaves as tiny particles called photons.
• Photons are virtual particles. The energy of the Photon is directly proportional to its frequency.
• The Photon’s energy is inverse proportion to its wavelength.
• The equation that relates the momentum and energy of the photons is

E = p x c

wherein

p = magnitude of the momentum

c = speed of light.

Thus, laws of the photoelectric effect were countered by understanding the properties of the photon.

Application of Photoelectric Effect

Laws of Photoelectric Effect questions proved beneficial in exploring various commercial uses of the effect.

Photocell, photoconductive devices, and solar cells are well-known applications of the photoelectric effect.

A photocell consists of two electrodes. A photosensitive cathode emits electrons when exposed to light, whereas an anode maintains a positive voltage. On emitting light on the cathode, electrons get attracted to the anode. The electrons in the tube flow from cathode to anode—the current stops when a beam of light incident on the cathode is interrupted. Cameras also use photocells as exposure metres. 

The photoconductive effect is closely related to the photoelectric effect, which increases the electrical conductivity of certain non-metallic materials such as cadmium sulphide when exposed to light. This effect can be pretty significant so that a minimal current in a device suddenly becomes quite significant when exposed to sunlight. 

Solar Cells consist of special silicon that acts as a battery when exposed to light. A single solar cell possesses the capacity of producing 0.6 volts. To obtain a higher voltage series of solar cells are joined in panels. Harnessing solar energy using the solar cell can help reach electricity to remote locations. The government has already laid plans to increase its usage.

Conclusion

Photoelectric Effect has been an essential pillar of the development of modern physics. Moreover, the laws of the photoelectric effect have played an instrumental role in inventing many valuable and helpful products. For example, photoelectric emission has played a significant role in developing products to harness solar energy. It also proved to be a revolutionary experiment for photography. The article looked into the laws of photoelectric effects, its detailed history of origin, and the experimental observations. The applications mentioned above of the photoelectric effect are one of the essential points to know for students. Ensure that you read the knowledge shared above in the article carefully.