Photoelectric

effectCourse: Diploma

Subject: Applied Science Physics

Unit: V

Chapter: II

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Photoelectric effect

• The phenomenon of emission of electrons from a

metallic surface by the use of light (or radiant) energy

is called photoelectric effect. The phenomenon was

discovered by Lenard. For photoelectric emission, the

metal used must have low work function, e.g., alkali

metals. Cesium is the best metal for photoelectric

effect.

1

Laws of Photoelectric effect

• The no. of electrons emitted per second i.e. photo

current is proportional to the intensity of incident

light.

• If frequency of incident radiation is below threshold

frequency, no photo electric emission will take place.

• The max. velocity or max. K.E of photoelectrons

depends on the frequency of radiation not on

intensity. K.E. Increases with the increase in

frequency.

Characteristics of Photoelectric effect

• (I) Effect of Intensity : Intensity of light means the

energy incident per unit area per second. For a given

frequency, if intensity of incident light is increased,

the photoelectric current increases and with decrease

of intensity, the photoelectric current decreases; but

the stopping potential remains the same.

• This means that the intensity of incident light affects

the photoelectric current but leaves the maximum

kinetic energy of photoelectrons unchanged.

• (ii) Effect of Frequency : When the intensity of

incident light is kept fixed and frequency is increased,

the photoelectric current remains the same; but the

stopping potential increases.

• If the frequency is decreased, the stopping potential

decreases and at a particular frequency of incident

light, the stopping potential becomes zero. This value

of frequency of incident light for which the stopping

potential is zero is called threshold frequency If the

frequency of incident light is less than the threshold

frequency no photoelectric emission takes place.

• Thus, the increase of frequency increases the

maximum kinetic energy of photoelectrons but leaves

the photoelectric current unchanged.

• (iii) Effect of Photo metal : When frequency and

intensity of incident light are kept fixed and photo

metal is changed, we observe that stopping

potentials versus frequency (v) graphs are parallel

straight lines, cutting frequency axis at different

points.

• (iv) Effect of Time : There is no time lag between the

incidence of light and the emission of photoelectrons.

EINSTEIN'S PHOTOELECTRIC

EQUATION• According to Plank's quantum theory, light is emitted

from a source in the forms of bundles of energy

called photons. Energy of each photon is .

• Einstein made use of this theory to explain how photo

electric emission takes place.

• According to Einstein, when photons of

energy fall on a metal surface, they transfer

their energy to the electrons of metal.

E h

E h

2

• When the energy of photon is larger than the minimum

energy required by the electrons to leave the metal

surface, the emission of electrons take place

instantaneously.

• The chance that an electron may absorb more then one

electron is negligible because the number of photons is

much lower than the electron.

• After absorbing the photon, an electron either leaves the

surface or dissipates its energy within the metal in such

a short interval that it has almost no chance to absorb

second photon.

• An increase in intensity of light source simply increases

the number of photon and the number of photo electrons

but no increase in the energy of photo electron.

• However, increase in frequency increases the energy of

photons and photo electrons. According to Einstein's

explanation of photoelectric emission, a photon of energy

'E' performs two operations:

1. Removes the electron from the surface of metal

2. Supplies some part of energy to move photo electron

towards anode

• Since minimum amount of energy to remove electron from

a surface is equal to work function, we can write Einstein

equation as:

• Energy Supplied = Energy Consumed in ejecting an

electron + maximum Kinetic energy of electron

• h f = KE + W

• KE = h f – W

• h fo = Wo

• Equations are identical and are known as Einstein's

photoelectric equations.

Photocell• Introduction:-

• A photocell is a practical application of the

phenomenon of photoelectric cell.

• Definition:-

• Photoelectric cell or photocell, device whose

electrical characteristics (e.g., current, voltage, or

resistance) vary when light is incident upon it.

OR

• The photo electric cell also known as phototube is an

electron tube in which the electrons initiating an

electric current originate by photo electric emission.

CONSTRUCTIPON OF PHOTOCELL

• Principle:-

• The working of photocell is based upon the

photoelectric effect.

• Construction:-

• It consists of a cathode and an anode in an evacuated

glass tube connected to appropriate terminals of the

battery as shown in the figure.

• The material of the cathode is selected to suit to the

frequency range of the incident radiation over which

the cell Is operated.

• For example, sodium or potassium cathode emits

photoelectrons fro visible light, cesium coated

oxidized silver emits electrons for the infrared light

and some other metals respond to ultraviolet

radiations.

• Working:-

• When light of frequency greater than threshold

frequency of the cathode falls on cathode plate,

photoelectrons are emitted.

• These are attracted towards the anode and due to this

flow of charges, current flows in the circuit.

• The number of electrons emitted depends upon the

intensity of light.

• When intensity of light is increased, the value of

current also increases.

• If light is switched off, the current flowing in the

circuit also stops.

Applications OF Photocell

• To count vehicles passing a road.

• To count items running on a conveyer belt.

• To open doors automatically in a building such as

banks or other commercial buildings or offices.

• To operate burglar alarms.

• To produce sound in movies.

• Photocells have myriad uses, especially as switches

and sensors.

• They are a common fixture in robotics, where they

direct robots to hide in the dark, or to follow a line or

beacon.

• Automatic lights that turn on when it gets dark use

photocells, as well as streetlights that switch on and

off according to whether it is night or day.

• They are used as timers to measure the speeds of

runners during a race.

• Photocells may be used in the place of variable

resistors and photovoltaic cells.

• Some circuit applications include light meters and

light controlled relays.

Examples.

1. Calculate the energy of a photon of blue light with a

frequency of 6.67 x 1014 Hz. (State in eV) [2.76eV]

2. Calculate the energy of a photon of red light with a

wavelength of 630 nm. [1.97eV]

3. Barium has a work function of 2.48 eV. What is the

maximum kinetic energy of the ejected electron if the

metal is illuminated by light of wavelength 450 nm?

[0.28 eV]

4. When a 350nm light ray falls on a metal, the maximum

kinetic energy of the photoelectron is 1.20eV. What is

the work function of the metal? [2.3 eV]

5. A photon has 3.3 x 10-19 J of energy. What is the

wavelength of this photon?

6. What is the energy of one quantum of 5.0 x 1014 Hz

light?

REFERENCE BOOKS AUTHOR/PUBLICATION

ENGINEERING PHYSICS S S PATEL (ATUL PRAKASHAN)

MODERN ENGINEERING

PHYSICSA S VASUDEVA

ENGINEERING PHYSICS K. RAJGOPALAN

Image Reference links

• http://0.tqn.com/y/chemistry/1/W/7/z/photoele

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