2ND PUC PHYSICS DUAL NATURE OF RADIATION AND MATTER
Work function :
The minimum energy needed by an electron to come out from a metal surface is called the work function of the metal. Energy (greater than the work function (φο) required for electron emission from the metal surface can be supplied by suitably heating or applying strong electric field or irradiating it by light of suitable frequency.
Photoelectric effect :
Photoelectric effect is the phenomenon of emission of electrons by metals when illuminated by light of suitable frequency. Certain metals respond to ultraviolet light while others are sensitive even to the visible light.
- Photoelectric effect involves conversion of light energy into electrical energy.
It follows the law of conservation of energy. - The photoelectric emission is an instantaneous process and possesses certain special features
Photoelectric current depends on
- the intensity of incident light.
- the potential difference applied between the two electrodes.
- the nature of the emitter material.
The stopping potential (Vo) depends on
- the frequency of incident light.
- the nature of the emitter material.
- for a given frequency of incident light, it is independent of its intensity.
Below a certain frequency (threshold frequency) ν0, characteristic of the metal, no photoelectric emission takes place, no matter how large the intensity may be.
The classical wave theory could not explain the main features of photoelectric effect. Its picture of continuous absorption of energy from radiation could not explain the independence of Kmax on intensity, the existence of νo and the instantaneous nature of the process. Einstein explained these features on the basis of photon picture of light. According to this, light is composed of discrete packets of energy called quanta or photons. Each photon carries an energy E (= hν) and momentum p (= h/λ), which depend on the frequency (ν) of incident light and not on its intensity. Photoelectric emission from the metal surface occurs due to absorption of a photon by an electron.
Einstein’s photoelectric equation:
Einstein’s photoelectric equation is in accordance with the energy conservation law as applied to the photon absorption by an electron in the metal. The maximum kinetic energy (1/2) mv2 max is equal to the photon energy (hν) minus the work function φ0 (= hν0) of the target metal:
This photoelectric equation explains all the features of the photoelectric effect. Millikan’s first precise measurements confirmed the Einstein’s photoelectric equation and obtained an accurate value of Planck’s constant h.
This photoelectric equation explains all the features of the photoelectric effect. Millikan’s first precise measurements confirmed the Einstein’s photoelectric equation and obtained an accurate value of Planck’s constant h.
Dual nature of radiation:
- Radiation has dual nature: wave and particle.
- The nature of experiment determines whether a wave or particle description is best suited for understanding the experimental result.
- The de Broglie wavelength (λ) associated with a moving particle is related to its momentum p as: λ = h/p.
- The dualism of matter is inherent in the de Broglie relation which contains a wave concept (λ) and a particle concept (p).
- The de Broglie wavelength is independent of the charge and nature of the material particle.
Electron diffraction experiments by Davisson and Germer, and by G. P. Thomson, as well as many later experiments, have verified and confirmed the wave-nature of electrons. The de Broglie hypothesis of matter waves supports the Bohr ’s concept of stationary orbits.
| Electron emission: | Definition of electron volt (eV) |
| Types of electron emission | |
| Photoelectric effect: | Hertz’s observations |
| Hallwachs’ and Lenard’s observations | |
| Phenomenon of Photoelectric effect | |
| Work function | |
| Threshold frequency and stopping potential | |
| Experimental setup to study Photoelectric effect: | Observations |
| Effect of intensity of light on photocurrent | |
| Effect of potential on photocurrent | |
| Effect of frequency of incident radiation on stopping potential | |
| Einstein’s photoelectric equation | |
| Particle nature of light: | The photon |
| Characteristics of photon | |
| Wave nature of matter: | De-Broglie hypothesis |
| De-Broglie wavelength | |
| Davisson and Germer experiment: | Schematic representation and theory |
| Wave nature of electrons on the basis of electron diffraction | |
| Numerical Problems |