Q. What is Einstein's photoelectric equation? Explain the laws of photoelectric effect with the help of this equation.

Photoelectric Effect : The emission of electron from a metal plate when illuminated by light of any other radiation of suitable wavelength or frequency is called photoelectric effect.

This is one of the experimental evidences of quantum particle nature of light or radiation. This effect was discovered by Hertz when ultraviolet light was allowed to fall on a zinc plate. He observed that electrons are emitted when light of certain frequency is incident on plate as shown in Fig. the electrons so emitted are known as photoelectrons and the current, so produced due to flow of photoelectrons is termed as photoelectric current.

Experimental Set up to Study Photoelectric Effect : Fig. Shows a typical setup used to study photoelectric effect. In an evacuated tube, two electrodes P1 and P2 are connected to an external electric circuit to provide desired potential difference. Electrode P1 behaves as anode and P2 behaves as cathode.

These two electrodes are connected to a microammeter, commutator, voltmeter and a battery as shown in Fig. Microammeter is used to measure current produced due to the flow of photoelectrons, thus current is not very big quantity so we use a very sensitive microammeter to measure it.

Commutator is used to change the polarity of potential on the plates i.e. P1 and P2 can be made positive or negative with respect to each other with the help of commutator. Potential difference between these two metal plates can be measured with the help of commutator. Potential difference between two metal plates can be measured with the help of voltmeter and the potential difference can be changed with the help of potential divider. Light of different intensities and different frequencies could be made to fall on the plate P1.

Einstein's Photoelectric Equation : According to this theory, light travels in the form of energy packages called photons. The velocity of these photons is equal to that of light and their energy is Hv, where 'h' is Planck's constant and v is the frequency f light. When light falls on a metal surface, a photon is completely absorbed by a single electron is the surface. A part of this energy is used in ejecting the electrons against the attraction of the rest part of energy is given to the electron as their kinetic energy so therefore according to energy conservation law.