Rescheduled CBSE Portion part alone described in that PPT. Kindly use it...

2. Wave theory of electromagnetic radiation
explained the phenomenon of interference,
diffraction and polarization.
On other hand, photo electric effect supported
particle’s nature of light.
Hence we assume dual nature of light.

3. Introduction
In 1897, Maxwell established electromagnetic theory
which unified all radiations like light and heat. Maxwell
established the wave theory of light. X-ray radiation was
also discovered during that time in 1895.

7. Free electrons in metals
cannot come out from the
surface due to force by positive
ions present in metals.
Electrons can come out of
the metal surface only if it has
sufficient energy to overcome
this attractive pull.

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

9. The energy gained by an electron
when it has been accelerated by a
potential difference of 1 volt.

12. Hertz’ Observation

13. Hertz’ Observation
The phenomenon of photo electric emission was
discovered in 1887 by Heinrich Hertz during his
electromagnetic wave experiment. In his experimental
investigation on the production of electromagnetic waves by
means of spark across the detector loop were enhanced
when the emitter plate was illuminated by ultraviolet light
from an arc lamp.

14. Lenard’s Observation

15. Lenard’s Observation
Lenard observed that when ultraviolet radiation were
allowed to fall on emitter plate of an evacuated glass tube
enclosing two electrodes, current flows. As soon as, the
ultraviolet radiations were stopped, the current flows also
stopped. These observations indicate that when ultraviolet
radiations fall on the emitter plate, electrons are ejected from it
which are attracted towards the positive plate by the electric
field.

16. Threshold Frequency
When ultra violet light fell on the emitter plate, no
electrons were emitted at all when the frequency of incident
light was smaller than a certain minimum value, called the
threshold frequency.
When the frequency of the incident light is smaller
than a certain minimum value, called the threshold
frequency no emission of electrons take place.

17. Experimental
study of photo
electric effect

19. Experimental outcome :
It showed that intensity of light has
linear relationship with photo electric
current at potential higher that
stopping potential.

21. Effect of intensity of light on
photocurrent

22. Effect of potential on photoelectric
current
For a given frequency of the incident
radiation, the stopping potential is independent
of its intensity.
Stopping Potential
The minimum negative (retarding) potential
Vo given to the plate A for which the photo

24. We take radiations of
different frequencies but of
same intensity.
For each radiation, we
study the variation of
photo electric current
against the potential
difference between the

25. The graph shows that
(i) the stopping potential V0 varies linearly with
the frequency of incident radiation for a given
photosensitive material.
(ii) there exists a certain minimum cut-off
frequency +γ0 for which the
stopping potential is zero.

26. These observations have two implications:
(i) The maximum kinetic energy of the photoelectrons
varies linearly with the frequency of incident radiation,
but is independent of its intensity.
(ii) For a frequency +γ of incident radiation, lower than
the cut-off frequency +γ0, no photoelectric emission is
possible even if the intensity is large.
This minimum, cut-off frequency +γ0, is called the
threshold frequency.

33. Laws of Photoelectric Emission
(i) For a given material and a given frequency of incident radiation,
the photoelectric current number of photoelectrons ejected per
second is directly proportional to the intensity of the incident light.
(ii) For a given material and frequency of incident radiation, saturation
current is found to be proportional to the intensity of incident
radiation, whereas the stopping potential is independent of its
intensity.

34. (iii) For a given material, there exists a certain minimum frequency of the
incident radiation below which no emissions of photoelectrons takes place.
This frequency is called threshold frequency.
Above the threshold frequency, the maximum kinetic energy of the emitted
photoelectron or equivalent stopping potential is independent of intensity of
incident light but depends only upon the frequency (or wavelength) of the
incident light.
(iv) The photoelectric emission is an instantaneous process. The time lag
between the incidence of radiations and emission of photoelectron is very
small, less than even 10-9 s.

35. Einstein Photoelectric Equation Energy Quantum of
Radiation,
Kmax = hγ – Ф0
where, hv = energy of photon and Ф = work-function
NOTE:
According to Planck’s quantum theory, light
radiations consist of tiny packets of energy called quanta.
One quantum of light radiation is called a photon which
travels with the speed of light.

36. Relation between Stopping potential (V0) and Threshold
frequency (γ0)
[∴Kmax = eV0]

37. Important Graphs related to Photoelectric Effect

41. The radiant energy has dual
aspects of particle and wave,
hence a natural question arises, if
radiation has a dual nature, why
not the matter.

42. Matter waves
In 1924, a French Physicist Louis de Broglie put
forward the bold hypothesis that moving particles should
possess wave like 64 properties under suitable conditions.
He reasoned this idea, from the fact, that nature is
symmetrical and hence the basic physical entities – matter
and energy should have symmetrical characters. If radiation
shows dual aspects, so should matter.

43. de Broglie’s wavelength of matter waves

45. de Broglie wavelength of an electron
When an electron of mass m and charge e is
accelerated through a potential difference V, then the
energy eV is equal to kinetic energy of the electron.

47. Example :
An electron is accelerated through a potential
difference of 100 volts. What is the de Broglie wavelength
associated with it? To which part of the electromagnetic
spectrum does this value of wavelength correspond?
Solution :
This wavelength corresponds X-rays