When electromagnetic radiations of suitable wavelength are incident on a metallic surface then electrons are
emitted, this phenomenon is called photo electric effect.
A graph between intensity of light and photoelectric current is found to
be a straight line as shown in figure. Photoelectric current is directly
proportional to the intensity of incident radiation. In this experiment
the frequency and retarding potential are kept constant.
THREE MAJOR FEATURES OF THE PHOTOELECTRIC EFFECT CANNOT BE
EXPLAINED IN TERMS OF THE CLASSICAL WAVE THEORY OF LIGHT
The light energy from any source is always an integral multiple of a smaller energy
value called quantum of light.hence energy
In 1905 Einstein made a remarkable assumption about the nature of light; namely, that, under some circumstances,
it behaves as if its energy is concentrated into localized bundles, later called photons. The energy E of
a single photon is given by
When light of intensity ? falls on a surface, it exerts force on that surface. Assume absorption and reflection
coefficient of surface be ‘a’ and ‘r’ and assuming no transmission.
Assume light beam falls on surface of surface area ‘A’ perpendicularly as shown
de-BROGLIE WAVELENGTH OF MATTER WAVE
A photon of frequency ? and wavelength ? has energy
J.J. Thomson suggested that atoms are just positively charge lumps of matter with electrons
embedded in them like raisins in a fruit cake. Thomson’s model called the ‘plum pudding’
model is illustrated in figure.
Rutherford suggested that; “ All the positive charge and nearly all the mass were concentrated in a very
small volume of nucleus at the centre of the atom. The electrons were supposed to move in circular
orbits round the nucleus (like planets round the sun). The electronstatic attraction between the two
opposite charges being the required centripetal force for such motion
In 1913, Prof. Niel Bohr removed the difficulties of Rutherford’s atomic model by the application of Planck’s
quantum theory. For this he proposed the following postulates
Kinetic and potential energies Kn and Un in nth orbit are given by
The Bohr model of hydrogen can be extended to hydrogen like atoms, i.e., one electron atoms, the
nuclear charge is +ze, where z is the atomic number, equal to the number of protons in the nucleus.
Definations valid for single electron system
Under normal conditions the single electron in hydrogen atom stays in ground state (n = 1). It is excited
to some higher energy state when it acquires some energy from external source. But it hardaly stays
there for more than 10–8 second.
Emission spectrum of hydrogen atom
According to Bohr when an atom makes a transition from higher energy level to a lower level it emits a photon
with energy equal to the energy difference between the initial and final levels. If Ei is the initial energy of the atom
EFFECT OF NUCLEUS MOTION ON ENERGY OF ATOM
It was discovered by ROENTGEN. The wavelength of x-rays is found between 0.1 Å
Production of x-rays by coolidge tube
The minimum wavelength corresponds to the maximum energy of the x-rays which in turn is equal to the
maximum kinetic energy eV of the striking electrons thus