Question

What is photoelectric effect? What are the laws of photoelectric effect? Explain this law given by Einstein.

Hint:

Photoelectric effect: Emission of electrons when light strikes a metal surface. Einstein’s explanation: \(K.E. = h f - W\), where \(h\) is Planck’s constant, \(f\) is frequency, and \(W\) is work function.

Answer 1

Photoelectric Effect:
The photoelectric effect is the phenomenon in which electrons are ejected from a material (usually metal) when light of sufficient frequency (or energy) strikes its surface. The emitted electrons are called photoelectrons. This effect was first observed by Heinrich Hertz in 1887, but it was Albert Einstein who provided the theoretical explanation in 1905, for which he was awarded the Nobel Prize in Physics.
Laws of Photoelectric Effect:
1. Emission of Electrons: When light of suitable frequency (above a certain threshold frequency) is incident on a metal surface, it causes the emission of electrons from the surface. The emitted electrons are called photoelectrons.
2. Threshold Frequency: There exists a minimum frequency of light, called the threshold frequency (\(f_0\)), below which no electrons are emitted, regardless of the intensity of light.
3. Effect of Light Intensity: The number of emitted photoelectrons depends on the intensity of the light. A higher intensity of light leads to more photoelectrons being emitted, but the kinetic energy of the photoelectrons remains unchanged.
4. Effect of Light Frequency: The kinetic energy of the emitted electrons depends on the frequency of the incident light. If the frequency is higher than the threshold frequency, the kinetic energy of the photoelectrons increases with the increase in frequency of the light.
Einstein's Explanation of the Photoelectric Effect:
Einstein proposed that light behaves as a stream of particles called photons, and each photon has energy given by: \[ E_{\text{photon}} = h f \] Where:
- \(E_{\text{photon}}\) is the energy of the photon,
- \(h\) is Planck’s constant,
- \(f\) is the frequency of the light.
When a photon strikes the metal surface, it transfers its energy to an electron. If the photon has energy greater than the work function (\(W\)) of the metal, the electron absorbs this energy and is ejected from the surface. The kinetic energy of the emitted photoelectron is given by: \[ K.E. = h f - W \] Where \(W\) is the work function, or the minimum energy required to eject an electron from the metal surface. Thus, Einstein’s equation explains the photoelectric effect by treating light as quantized packets of energy.