DUAL NATURE OF RADIATION AND MATTER���BY�HAREESH KUMAR VK�PGT PHYSICS� JNV BIDAR �
ELECTRON EMISSION
THERMIONIC EMISSION:
FIELD EMISSION
PHOTO-ELECTRIC EMISSION
HERTZ’S OBSERVATIONS�
High voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light
HALLWACHS’ AND LENARD’S OBSERVATIONS
Lenard observed that when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes , current flows in the circuit . As soon as the ultraviolet radiations were stopped, the current flow also stopped
Hallwachs, observed that the negatively charged zinc plate lost its charge when it was illuminated by ultraviolet light
EXPERIMENTAL STUDY OF PHOTOELECTRIC EFFECT�
EFFECT OF INTENSITY OF LIGHT ON PHOTOCURRENT� �
Number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation
EFFECT OF POTENTIAL ON PHOTOELECTRIC CURRENT�
The maximum value of the photoelectric current is called saturation current.
The minimum negative (retarding) potential V0 given to the plate A for which the photocurrent stops or becomes zero is called the cut-off or stopping potential.
Photoelectric current is zero when the stopping potential is sufficient to repel even the most energetic photoelectrons, with the maximum kinetic energy
Kmax = e V0
For a given frequency of the incident radiation, the stopping potential is independent of its intensity
EFFECT OF FREQUENCY OF INCIDENT RADIATION ON STOPPING POTENTIAL�
PHOTOELECTRIC EFFECT AND WAVE THEORY OF LIGHT
The wave picture is unable to explain the most basic features of photoelectric emission.
EINSTEIN’S PHOTOELECTRIC EQUATION�
Radiation energy is built up of discrete units – the so called quanta of energy of radiation. Each quantum of radiant energy has energy hν
Kmax = hν – φ0
Photoelectric emission is possible only if h ν > φ0
This is an important result. It predicts that the V0 versus ν curve is astraight line with slope = (h/e),
PARTICLE NATURE OF LIGHT: THE PHOTON�
(i) In interaction of radiation with matter, radiation behaves as if it is made up of particles called photons.
(ii) Each photon has energy E (=hν) and momentum p (= h ν/c), and speed c.
(iii) All photons of light of a particular frequency ν, or wavelength ν, have the same energy E and momentum p , whatever the intensity of radiation may be.
WAVE NATURE OF MATTER�
De Broglie proposed that the wave length λ associated with a particle of momentum p is given as
λ =h/p = h/mv
known as the de Broglie relation and the wavelength λ of the matter wave is called de Broglie wavelength
WAVE LENGTH OF THE ELECTRON
HEISENBERG’S UNCERTAINTY PRINCIPLE.
According to the principle, it is not possible to measure both the position and momentum of an electron (or any other particle) at the same time exactly.
DAVISSON AND GERMER EXPERIMENT�
The wave nature of electrons was first experimentally verified by C.J. Davisson and L.H. Germer
The de Broglie wavelength λ associated with electrons for V = 54 V is given by
Thus, there is an excellent agreement between the theoretical value and the experimentally obtained value of de Broglie wavelength. Davisson- Germer experiment thus strikingly confirms the wave nature of electrons and the de Broglie relation
wave properties of electrons have been utilised in the design of electron microscope