1) The document discusses the photoelectric effect and its implications, including Einstein's explanation using light quanta. 2) It describes early experiments by Hertz, Lenard and others that showed light behaving as particles rather than waves. 3) Key points are that the maximum kinetic energy of emitted electrons depends on light frequency, not intensity, challenging classical wave theory.

1. October 29
PHYSICS 2012
ubmitted by: - Swarup Kumar Boro
ass: - XII

2. Contents
1. Photoelectric Effect
2. Laws of Photoelectric Emission
3. The Classical Wave Explanation
4. Hertz’s Observations
5. Lenard’s Observations
6. Einstein’s Photoelectric Equation:
7. Application of Photoelectric Effect:
8. de Broglie wave:
9. Davisson and Germer Experiment

3. Photoelectric Effect
The photoelectric effect posed a significant challenge to
the study of optics in the latter portion of the 1800s. It
challenged the classical wave theory of light, which was
the prevailing theory of the time. It was the solution to
this physics dilemma that catapulted Einstein into
prominence in the physics community, ultimately earning
him the 1921 Nobel Prize.
Though originally observed in 1839, the photoelectric
effect was documented by Heinrich Hertz in 1887 in a
paper to the Annalen der Physik. It was originally called
the Hertz effect, in fact, though this name fell out of use.

4. When a light source (or, more generally, electromagnetic
radiation) is incident upon a metallic surface, the surface
can emit electrons. Electrons emitted in this fashion are
called photoelectrons (although they are still just
electrons). This is depicted in the image to the right.
Setting Up the Photoelectric
Effect
To observe the photoelectric effect, you create a
vacuum chamber with the photoconductive metal at one
end and a collector at the other. When a light shines on
the metal, the electrons are released and move through the
vacuum toward the collector. This creates a current in the
wires connecting the two ends, which can be measured
with an ammeter. (A basic example of the experiment can
be seen by clicking on the image to the right, and then
advancing to the second image available.)
By administering a negative voltage potential (the
black box in the picture) to the collector, it takes more
energy for the electrons to complete the journey and
initiate the current. The point at which no electrons make
it to the collector is called the stopping potential Vs, and

5. can be used to determine the maximum kinetic energy
Kmax of the electrons (which have electronic charge e) by
using the following equation:
Kmax = eVs
It is significant to note that not all of the electrons will
have this energy, but will be emitted with a range of
energies based upon the properties of the metal being
used. The above equation allows us to calculate the
maximum kinetic energy or, in other words, the energy of
the particles knocked free of the metal surface with the
greatest speed, which will be the trait that is most useful
in the rest of this analysis.

6. Laws of Photoelectric Emission
i) For a given substance, there is a minimum value of
frequency of incident light called threshold frequency
below which no photoelectric emission is possible,
howsoever, the intensity of incident light may be.
ii) The number of photoelectrons emitted per second
(i.e. photoelectric current) is directly proportional to
the intensity of incident light provided the frequency
is above the threshold frequency.
iii) The maximum kinetic energy of the photoelectrons is
directly proportional to the frequency provided the
frequency is above the threshold frequency.
iv) The maximum kinetic energy of the photoelectrons is
independent of the intensity of the incident light.
v) The process of photoelectric emission is
instantaneous. I.e. as soon as the photon of suitable
frequency falls on the substance, it emits
photoelectrons.
vi) The photoelectric emission is one-to-one. i.e. for
every photon of suitable frequency one electron is
emitted.

7. The Classical Wave Explanation
In classical wave theory, the energy of electromagnetic
radiation is carried within the wave itself. As the
electromagnetic wave (of intensity I) collides with the
surface, the electron absorbs the energy from the wave
until it exceeds the binding energy, releasing the electron
from the metal. The minimum energy needed to remove
the electron is the work function phi of the material. (Phi
is in the range of a few electron-volts for most common
photoelectric materials.)
Three main predictions come from this classical
explanation:
1. The intensity of the radiation should have a
proportional relationship with the resulting maximum
kinetic energy.
2. The photoelectric effect should occur for any light,
regardless of frequency or wavelength.
3. There should be a delay on the order of seconds
between the radiation’s contact with the metal and
the initial release of photoelectrons.

8. The Experimental Result
By 1902, the properties of the photoelectric effect were
well documented. Experiment showed that:
1. The intensity of the light source had no effect on the
maximum kinetic energy of the photoelectrons.
2. Below a certain frequency, the photoelectric effect
does not occur at all.
3. There is no significant delay (less than 10-9 s)
between the light source activation and the emission
of the first photoelectrons.

9. Hertz’s ObservatiOns
The phenomenon of photoelectric effect was discovered
by Heinrich Hertz in 1887. While performing an
experiment for production of electromagnetic waves by
means of spark discharge, Hertz observed that sparks
occurred more rapidly in the air gap of his transmitter
when ultraviolet radiations was directed at one of the
metal plates. Hertz could not explain his observations but
other scientists did it. They arrived at the conclusion that
the cause was the emission of electron from metal plate
due to incidence of high frequency light. This is
photoelectric effect.

10. Lenard’s ObservatiOns
Phillip Lenard observed that when ultraviolet radiations
were made incident on the emitter plate of an evacuated
glass tube enclosing two metal plates (called electrodes),
current flows in the circuit, but as soon as ultraviolet
radiation falling on the emitter plate was stopped, the
current flow stopped. These observations indicate that
when ultraviolet radiations fall on the emitter (cathode)
plate C, the electrons are ejected from it, which are
attracted towards anode plate A. The electrons flow
through the evacuated glass tube, complete the circuit and
current begins to flow in the circuit. Then Hallwach’s and
Lenard studied the phenomenon in detail.
Hallwach’s studied further by taking a zinc plate and an
electroscope. The zinc plate was connected to an
electroscope. He observed that :
(i) When an uncharged zinc plate was irradiated by
ultraviolet light, the zinc plate acquired positive charge.
(ii) When a positively charged zinc plate is illuminated by
ultraviolet light, the positive charge of the plate was
increased.
(iii) When a negatively charged zinc plate was irradiated
by ultraviolet light, the zinc plate lost its charge.

11. All these observations show that when ultraviolet light
falls on zinc plate, the negatively charged particles
(electrons) are emitted.
Further study shows that different metals emit electrons
by different electromagnetic radiations. For example the
alkali metals (e.g., sodium, cesium, potassium etc.) emit
electrons when visible light is incident on them. The
heavy metals (such as zinc, cadmium, magnesium etc.)
emit electrons when ultraviolet radiation is incident on
them.
Cesium is the most sensitive metal for photoelectric
emission. It can emit electrons with less-energetic infrared
radiation.
In photoelectric effect the light energy is converted into
electrical energy.

12. einstein’s PHOtOeLectric
Equation:
When a photon of energy hν falls on a metal surface,
the energy of the photon is absorbed by the electron and is
used in two ways:
i) A part of energy is used to overcome the surface
barrier and come out of the metal surface. This part
of the energy is called ‘work function’ (Ф =
hν0).
ii) The remaining part of the energy is used in giving a
velocity ‘v’ to the emitted photoelectron. This is
equal to the maximum kinetic energy of the
photoelectrons ( ½ mv2max ) where ‘m’ is mass of the
photoelectron.
According to law of conservation of energy,
hν = Ф + ½ mv2max
= hν0 + ½ mv2max
½ mv2max = h (ν - ν0)

13. Application of Photoelectric
Effect:
1. Automatic fire alarm
2. Automatic burglar alarm
3. Scanners in Television transmission
4. Reproduction of sound in cinema film
5. In paper industry to measure the thickness of paper
6. To locate flaws or holes in the finished goods
7. In astronomy
8. To determine opacity of solids and liquids
9. Automatic switching of street lights
10. To control the temperature of furnace
11. Photometry
12. Beauty meter – To measure the fair complexion of
skin
13. Light meters used in cinema industry to check the
light
12. Photoelectric sorting

14. de Broglie wave:
According to de Broglie, a moving material particle
can be associated with a wave. i.e. a wave can guide
the motion of the particle.
The waves associated with the moving material
particles are known as de Broglie
waves or matter waves.
Expression for de Broglie wave:
According to quantum theory, the energy of the
photon is
According to Einstein’s theory, the energy of the photon
is
E=mc2
So,

15. Or
Where p = mc is momentum of a photon
If instead of a photon, we have a material particle of mass
m moving with velocity v, then the equation becomes
. This is the expression for de Broglie
wavelength.
Conclusion:
i) de Broglie wavelength is inversely proportional to the
velocity of the particle. If the particle moves faster,
then the wavelength will be smaller and vice versa.
ii) If the particle is at rest, then the de Broglie
wavelength is infinite. Such a wave cannot be
visualized.
iii) de Broglie wavelength is inversely proportional to the
mass of the particle. The wavelength associated with
a heavier particle is smaller than that with a lighter
particle.

16. iv) de Broglie wavelength is independent of the charge
of the particle.
Davisson and Germer Experiment
A beam of electrons emitted by the electron gun is
made to fall on Nickel crystal cut along cubical axis at a
particular angle.
The scattered beam of electrons is received by the
detector which can be rotated at any angle.
The energy of the incident beam of electrons can be
varied by changing the applied voltage to the electron
gun.
Intensity of scattered beam of electrons is found to
be maximum when angle of scattering is 50° and the
accelerating potential is 54 V
Electron diffraction is similar to X-ray diffraction.
Bragg’s equation 2dsinθ = nλ gives
λ = 1.65 Å