What is photoelectric effect?

The ejection of electrons from a metal plate when illuminated by light or any other electromagnetic radiation of suitable wavelength (or frequency) is called photoelectric effect. Although these electrons are not different from all other electrons, it is customary to call them as photoelectrons and the corresponding current as photoelectric current or photo current. Metals like cadmium, zinc, magnesium etc show photoelectric emission for ultraviolet light while some alkali metals lithium, sodium, caesium respond well even to larger wavelength radiation like visible light. The materials which eject photoelectrons upon irradiation of electromagnetic wave of suitable wavelength are called photosensitive materials.

E M B I B E

Physics Standard 12 Vol II>Chapter NA - Dual Nature of Radiation and Matter>EVALUATION>Q 14

EASY

12th Tamil Nadu Board

IMPORTANT

Earn 100

Define stopping potential.

Important Points to Remember in Chapter NA - Dual Nature of Radiation and Matter from Tamil Nadu Board Physics Standard 12 Vol II Solutions

1. Photoelectric effect and Einstein's photoelectric equation:

(i) Particle is a material object which is considered as a tiny concentration of matter (localized in space and time) whereas wave is a broad distribution of energy in (not localized space and time).

(ii) The liberation of electrons from any surface of a substance is called electron emission.

(iii) The minimum energy required by an electron to escape from the metal surface is called the work function of the metal.

(iv) One electron volt is the energy gained by an electron when it has been accelerated by a potential difference of $1 Volt$ , so that $1 e V = 1.602 \times 10^{- 19} J$ .

(v) There are three ways to provide energy to an electron so that it escapes the metal surface: Thermionic emission, Field emission, Photoelectric emission.

(vi) Some photosensitive substances emit electrons called as photoelectrons when they are illuminated by light by a phenomenon called as photoelectric effect. A stream of photoelectrons is called photocurrent.

(vii) The minimum frequency of the incident photon for photoelectric effect to happen is called threshold frequency, and the energy of the photon at this frequency is equal to the work function of the metal.

(viii) The photocurrent is directly proportional to the intensity of incident radiation, and it also increases with the collector plate potential to a certain limit until it reaches the saturating current.

(ix) The minimum negative (retarding) potential given to the collector plate for which the photocurrent stops or becomes zero is called the cut-off or stopping potential. The maximum kinetic energy of the photoelectrons and the stopping potential are related as $K E_{m a x} = e V_{s}$ .

(x) Einstein’s photoelectric equation: $K E_{m a x} = h ν - ϕ$ , where $K E_{m a x}$ is the maximum kinetic energy of the released photoelectrons, $ν$ is the frequency of the incident light and $ϕ$ is the work function of the metal. The above equation can also be written as: $e V_{s} = h ν - ϕ$ . At threshold frequency, $h ν_{t h} = ϕ$ .

(xi) When intensity is increased keeping the frequency constant, the number of photons per unit time gets increased resulting in increased photocurrent.

2. Matter waves:

(i) Energy of a photon, $E = h ν$ or $\frac{h c}{λ}$ . Momentum of a photon, $P = \frac{h}{λ}$ .

(ii) The de-Broglie wavelength is given by, lambda $λ = \frac{h}{P}$ or $λ = \frac{h}{m u}$ or $\frac{h}{\sqrt{2 m K E}}$

(iii) The de-Broglie wavelength of an electron accelerated by a potential of $V$ is given by, $λ = \frac{1.227}{\sqrt{V}}$ .

3. Heisenberg’s uncertainty principle:

According to Heisenberg’s uncertainty principle, it is not possible to measure both the position and momentum of a particle at the same time exactly. The uncertainty in position ( $Δ x$ ) and in momentum ( $Δ P$ ) are related as $Δ x Δ P = \frac{h}{2 π}$ .

4. Diffraction of electrons:

Discovery of diffraction of electrons by C.J. Davisson and L.H. Germer lead to verification of the wave nature of electrons. In this experiment, electrons are bombarded on a metal which scatters them. The scattered electrons are collected by a collector and it is observed that the intensity of the scattered electrons is dependent of the subtle variations in the position of the collector proving the phenomenon of diffraction in electrons.

Learn and Prepare for any exam you want !

|

|

|

|

|

|

|

|