The document discusses the photoelectric effect, which is the phenomenon where electrons are ejected from matter after absorbing electromagnetic radiation such as light. It describes key aspects of the effect such as photons, the properties of photons, what photoelectric effect is, who discovered it, devices based on it, and laws governing it. The document also discusses Einstein's explanation of the photoelectric effect using the photon model of light and how this helped establish the particle nature of light.

1. viva-voce questions
viva-voce questions
PHOTO-ELECTRIC EFFECT
PHOTO-ELECTRIC EFFECT

2. What are photons?
A packet or bundle of energy is called a photon.
E = hν = hc
Que.no-1
A photon is an elementary particle that is a quantum of the electromagnetic field,
including electromagnetic radiation such as light and radio waves, and the force
carrier for the electromagnetic force.
OR
Energy of a photon is,
λ
where,
h is the Planck’s constant,
ν is the frequency of the radiation or photon,
c is the speed of light (e.m. wave),
λ is the wavelength.

3. What are the properties of photons?
Que.no-2
A photon travels at a speed of light c in vacuum. (i.e. 3 x 10-8 m/s)
It has zero rest mass. i.e. the photon can not exist at rest.
Photons travel in a straight line.
Photons are electrically neutral.
Photons may show diffraction under some conditions.
Photons are not deviated by magnetic and electric fields.
Energy of a photon depends upon frequency of the photon; so the energy of the
photon does not change when photon travels from one medium to another.
The kinetic mass of a photon is,
The momentum of a photon is,
m= E/c = h/cλ
p= E/c = h/λ
2

4. What are Photoelectric effect?
Que.no-3
The photoelectric effect is a phenomenon in which electrons are ejected from the
surface of a metal when light is incident on it.
These ejected electrons are called photoelectrons.
Principle of Photoelectric Effect-
The "law of conservation of energy" forms the basis for the photoelectric effect.

5. Who discovered the photoelectric effect?
Que.no-4
The phenomenon of the photoelectric effect was discovered by Heinrich Hertz in 1887.

6. Which popular device is based on the photoelectric effect?
Que.no-5
One of the common devices based on this effect is the photoelectric cell or
photodiode.

7. What is the importance of the photoelectric effect?
Que.no-6
The study of the photoelectric effect has led to expanding our understanding of the
quantum nature of light and electrons. It has further influenced the formation of the
concept of wave-particle duality. The photoelectric effect is also widely used to
investigate electron energy levels in the matter.

8. What are the characteristics Of Photoelectric Effect?
Que.no-7
The threshold frequency varies with material, it is different for different materials.
The photoelectric current is directly proportional to the light intensity.
The kinetic energy of the photoelectrons is directly proportional to the light
frequency.
The stopping potential is directly proportional to the frequency and the process is
instantaneous.

9. What factors affecting Photoelectric Effect ?
Que.no-8
The intensity of incident radiation.
A potential difference between metal plate and collector.
Frequency of incident radiation.

10. Discuss the laws of Photoelectric Effect ?
Que.no-9
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.
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.
The maximum kinetic energy of the photoelectrons is directly proportional to the
frequency provided the frequency is above the threshold frequency.
The maximum kinetic energy of the photoelectrons is independent of the
intensity of the incident light.

11. The process of photoelectric emission is instantaneous. i.e. as soon as the photon
of suitable frequency falls on the substance, it emits photoelectrons.
The photoelectric emission is one-to-one. i.e. for every photon of suitable frequency
one electron is emitted.

12. What are the conditions for Photoelectric Effect ?
Que.no-10
It is the minimum frequency of the incident light or radiation that will produce a
photoelectric effect i.e. ejection of photoelectrons from a metal surface is known
as threshold frequency for the metal.
It is constant for a specific metal but may be different for different metals.
Threshold Frequency-
Threshold wavelength-
During the emission of electrons, a metal surface corresponding to the greatest
wavelength to incident light is known threshold wavelength.
Work Function or Threshold Energy
The minimal energy of thermodynamic work that is needed to remove an electron
from a conductor to a point in the vacuum immediately outside the surface of the
conductor is known as work function/threshold energy.

13. What is the formula of Photoelectric Effect ?
Que.no-11
According to Einstein’s explanation of the photoelectric effect :
The energy of photon = energy needed to remove an electron + kinetic energy of the emitted electron
i.e. hν = W + E
Where,
h is Planck’s constant.
ν is the frequency of the incident photon.
W is a work function.
E is the maximum kinetic energy of ejected electrons: 1/2 mv².

14. What is dual Nature of radiation and matter ?
Que.no-12
Wave theory of electromagnetic radiations explained the phenomenon of
interference, diffraction and polarization.
On the other hand, quantum theory of e.m. radiations successfully explained the
photoelectric effect, Compton effect, black body radiations, X- ray spectra, etc.
Louis de Broglie suggested that the particles like electrons, protons,
Thus, radiations have dual nature. i.e. wave and particle nature.
neutrons, etc have also dual nature. i.e. they also can have particle as
well as wave nature.
"In no experiment, matter exists both as a particle and as a wave simultaneously. It
is either the one or the other aspect. i.e. The two aspects are complementary to
each other."

15. What is De-Broglie wave ?
Que.no-13
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 , E = hν = hc
λ
According to Einstein’s theory, the energy of the photon is, E = mc²
If instead of a photon, we have a material particle of mass m moving with velocity v,
then the equation becomes,
λ = h
mv

16. What are the applications of photoelectric effect ?
Que.no-14
Automatic fire alarm
Automatic burglar alarm
Reproduction of sound in cinema film
To determine opacity of solids and liquids
Automatic switching of street lights
Light meters used in cinema industry to check the light
sed in photomultipliers to detect low levels of light.
Used in video camera tubes in the early days of television.
Night vision devices are based on this effect.
Motion and Position Sensors
Used to generate electricity in Solar Panels.
X-Ray Photoelectron Spectroscopy

17. Plott the graph Photoelectric current V/s Retarding potential for different voltages ?
Que.no-15

18. Plott the graph Photoelectric current V/s Retarding potential for different intensities ?
Que.no-16

19. Plott the graph Electron current V/s Light Intensity ?
Que.no-17

20. Plott the graph Stopping potential V/s frequency ?
Que.no-18

21. Plott the graph Electron current V/s Light frequency ?
Que.no-19

22. Plott the graph Electron kinetic energy V/s Light frequency ?
Que.no-20

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