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Electrons and Photons

Electrons

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Electron

Experiments with a Crookes tube first demonstrated the particle

nature of electrons. In this illustration, the profile of the Maltese-

cross-shaped target is projected against the tube face at right by a

beam of electrons.[1]

Composition Elementary particle[2]

Generation First

Interactions Gravity, Electromagnetic, Weak

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Symbol e−, β−

Antiparticle Positron (also called antielectron)

Theorized Richard Laming (1838–1851),[3]

G. Johnstone Stoney (1874) and others.[4][5]

Discovered J. J. Thomson (1897)

Mass 9.10938291(40)×10−31 kg

Electric charge

−1.602176565(35)×10−19 C

Magnetic moment −1.00115965218076(27) μB[7]

Spin 1⁄2

Photon

a photon is a discrete bundle (or quantum) of

electromagnetic (or light) energy. 3

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Photo electric effect

Electrons are present on the surface of conductor.

These electrons are loosely bounded with the 4

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nucleus. In few metals these electrons are released

from its surface when the metal is expose to light.

This effect is known as photo electric effect.

“Phenomenon of emission of electrons by metal

surface under the action of light is called as photo-

electric effect. The emitted electrons in this process

are called as photo-electrons.” Alkali metals like

sodium, potassium shows this effect.

1. Frequency of incident radiation.

1. Initially P.D. & intensity of radiation are kept

constant.

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It is observed that, at low frequency, photo-

electric current cannot flow.

This certain minimum frequency at and above

which photoelectric current starts is called as

threshold frequency ( ).

Threshold frequency is differs for different

metals. Wavelength of light corresponding to

threshold frequency is called as threshold

wavelength (λ0)

2. Now, frequency (greater than threshold

frequency) & potential difference are kept

constant.

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It is observed that photoelectric current

increases as intensity of incident radiation

increases.

Photoelectric current is directly proportional to

intensity of incident radiations.

3. Now frequency (greater than threshold

frequency) & intensity incident radiation are

kept constant

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It is observed the photo-electric current stops at

certain -ve potential. This value of potential is

called as stopping potential (Vs).

It is also observed that value of stopping

potential increases as the frequency of incident

radiation increases.

The relation between kinetic energy (K.E.) and

stopping potential (Vs) is K.E.=Vs.e

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4. As soon as light radiation incident on photo

sensitive material, electron emission starts and

current flows. That means photoelectric effect is

an instantaneous process.

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Characteristics of photo electric effect.

1. Photo-electric effect occurs only when the

frequency of incidence radiation is greater than

certain minimum frequency called as threshold

frequency (v0).

2. The number of electrons emitted per second is

directly proportional to intensity of incident

radiation.

3. The maximum K.E. of photo-electrons depends

upon frequency of incident radiation K.E.

increases with increase in frequency.

4. Photo-electric effect is as instantaneous

process.

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Einstein’s photo-electric equation.

Definition:

“Minimum amount of light energy required to

release an electron from metal surface is called as

photo electric work function.”

If frequency of incident radiation is decreased upto

threshold frequency then, kinetic energy becomes

zero. i.e. whenν = ν0, K.E. = 0

∴ hν0 = W0

Substitute this value in equation

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1. From this relation it is clear that for emission of

electrons frequent n of incident radiation n must

be greater than threshold frequency.

2. From above equation it is also clear that, as

frequency of incident radiations increases K.E.

of emitted electrons increases.

3. The intense beam of light contains maximum

number of photons. Due to this, number of

collisions are more & number of electrons

emitted per second are more. That is emission

of electrons is directly proportional to intensity

of incident radiations.

4. Photoelectric effect is an instantaneous

process.

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Photoelectric cell: -

Cathode is a concave shape electrode & coated

with photo sensitive material like sodium cesium

etc. Anode is a platinum rod. There are two pins

from t electrodes to connect the cell into external 13

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circuit. There is a non conducting base to the

photocell.

Applications of photo cell

Photocell is used in the following applications:

1. Exposure meter:

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2. Lux meter :

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3. Burglar alarm:

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4. Production of sound recorded on cine film:

Sound is recorded on the part of cine film.

When light from this part incident on photocell

from this part, due to change in intensity,

magnitude of current changes. Using this

current, a sound is produced.

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