Dual Nature of Radiation and Matter

Second Year Physics study material

Modern physics possess five important chapters

1. Dual nature of Radiation and Matter

2. Atoms

3. Nuclei

4. Semi conductor electronics

5. Communication systems

Blue print of marks for PUC and CBSE board are as follows

CBSE Board:

For PUC board Karnataka

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Dual nature of

radiation and matter

Atoms and nuclie

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Dual nature of

radiation and

matter

Atoms and nuclie


Modern physics possess five important chapters

1. Dual nature of Radiation and Matter

and CBSE board are as follows

For PUC board Karnataka

1110

3

Atoms and nuclie Electronic devices Communication

systems

Marks weightage

1110

3

Atoms and nuclie Electronic devices Communication

systems

Marks weightage

Second Year Physics study material 2015

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Marks weightage

Marks weightage


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Dual Nature of Radiation and Matter

1. Write a short note on Electronic emissions?

A) Electrons bound to nucleus in an atom. Some energy should be given to break the

bond between nucleus and electron. The minimum amount of energy should be

supplied to set free the electron from nucleus influence is called work function.

Energy can be supplied by means of three ways to set free the electron

1. Thermionic emission 2. Field emission 3. Photo-electric emission

Thermionic emission: By suitably heating, sufficient thermal energy is Supplied to

free electrons and enable them to come out of metal.

Field emission: By applying strong electric field to the metal, electrons can be pulled

out of the metal

Photo electric emission: When light of suitable frequency suitably illuminates

surface, electrons are emitted from metal surface. Generated electrons are called photo

electrons

2. What is photo electric effect?

A) The phenomenon of emission of electrons from preferably metal surfaces, Exposed to

light energy of suitable frequency is known as Photo electric effect .

3. What are Hallwach and Lenard observations regarding photo electric effect?

A) Hallwach and Lenard studied how photo current varies with

1. Collector plate potential

2. Frequency of incident light

3. Intensity of incident light.

Lenard observed , when ultraviolet radiation were allowed to fall on emitter plate of

an evacuated glass tube enclosing two electrodes , current flows in the circuit. As

soon as ultraviolet radiation stopped, current flow will also stop.

Hallwach still observed when negatively charged zinc plate connected to emitter plate,

it loosed its charge. When neutral zinc plate connected to emitter plate, it gains

positive charge when positively charged zinc plate connected to emitter plate, its

positive charge further enhanced. By those observations, he concluded negatively

charged particles were emitted from zinc plate under the action of ultraviolet light.


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After the discovery of electron, it is evident that incident light causes electrons to emit

from plate.

4. Which metals emit photo electrons when incident by Ultraviolet light and

Visible light?

A) Metals like Zinc, cadmium, Magnesium respond only to ultraviolet light. Alkali

metals like lithium, sodium, potassium, cesium and Rubidium are sensitive to visible

light.

5. What is photo electric effect? How to study photo electric effect

experimentally?

A) The phenomenon of emission of electrons from preferably metal surfaces, Exposed to

light energy of suitable frequency is known as Photo electric effect .

The experimental setup of photo electric effect is as shown in figure. It consists of a

tube having two electrodes A and C in vacuum. The electrode C is called cathode. it is

photo sensitive material. When light of suitable frequency incident on the plate C,

electrons are emitted.

The Electrode A is anode; the electrons emitted from cathode will be received at this

plate. The tube has a side window made of quartz covered with filter, through which

the incident light of desired wavelength enters the tube and falls on a photosensitive

plate C.

These Plates are connected through a battery through a suitable switch. Plate A can

be brought to positive or negative potential with respect to cathode C.

Electrons are emitted when ultraviolet radiations are made to fall on photosensitive

plate C. These electrons are accelerated towards Anode A when it is at positive

potential with respect to cathode. This flow of electrons from cathode to anode creates


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electric current to flow in the circuit. This is known as photo electric current. Photo

electric current is measured with micro ammeter connected in series with circuit. The

potential between two plates are measured with the help of Ammeter.

6. Explain briefly the effect on photo electric current with variation in

1. Intensity of incident light 2. Potential difference 3. Effect of frequency?

A) Effect of intensity of incident light on photo electric current :

Photo electric current (number of electrons emitted

per second) is directly proportional to the intensity of

incident radiation, provided the frequency of incident

radiations is greater than threshold frequency

Let a constant potential maintained between plates,

when light of suitable frequency falls on it , photo

electrons are emitted. As the intensity of light

increases by keeping the frequency constant, photo

electric current increases. The relationship is linear.

Effect of potential on photo electric current:

When positive potential of

plateA increases, photo electric

current increases. When positive

potential of plate is further

increased, photo electric current

saturates at a particular value

called Saturation current

When intensity of light

changes, saturation current

value changes with change in

positive potential of plate.

When potential of plate A becomes negative, most of electrons will repelled and

photo electric current decreases and become zero at certain point. Although

intensity of light changes, Always electric current becomes zero at a single point.

The Negative voltage at the photo electric current becomes zero is called Stopping

potential . Stopping potential is independent of intensity of incident light.


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Conclusions from this:

1. The intensity of incident radiation does not affect the stopping potential

2. Maximum kinetic energy of photoelectrons does not depend upon intensity of

incident radiation.

Effect of frequency of incident light:

By keeping intensity of light as constant, using various frequencies of light and

calculated stopping potentials. Saturation current is same for every frequency of light,

as it does not depend on frequency , it only

depends on intensity of light.

Stopping potential changes with frequency of

light. Stopping potential value becomes more

negative for high frequencies. Stopping potential

increases linearly with increase in frequency.

Conclusion:

1. Saturation current does not depend on

frequency of incident light.

2. Maximum kinetic energy of photoelectrons

depends on frequency of incident radiation.

7. What are Laws of Photo electric emission?

1. For a given substance , there is a minimum frequency of incident light called

threshold frequency below which no photo electric emission take place ,

whatever the intensity of light may be

2. The number of photo electrons emitted per second is directly proportional to the

intensity of incident light, provided the frequency of incident light is greater

than threshold frequency

3. The maximum kinetic energy of photo electrons increases with the increase in

the frequency of the incident light, provided the frequency of incident light is

greater than threshold frequency

4. The process of photoelectric emission is instantaneous.

8. Does Wave theory of light can explain observations of Photoelectric effect?

A) Wave theory failed to explain observations made by Photoelectric effect.

According to wave picture of light , the free electrons at the surface of metal (over

which the beam of radiation falls) absorb the radiant energy continuously. The

greater the intensity of radiation, the greater is the amplitude of electric and magnetic

fields.


Consequently, the greater the intensity the gr

each electron. In this picture the

surface is then expected to increase with intensity.

Also, no matter what the frequency of ra

impart enough energy to the electrons, so they

escape from the metal surface. A threshold frequency should not exist.

In the wave picture, there should be enough time delay

metallic surface and the emission of electrons from

observed that electrons are emitted from metallic surface as soon as light is incident

on it.

9. Write a short note on Einstein Expl

A) According to Einstein, Energy interacts with matter in the form of packets of energy

called quanta. Later this quanta is termed as a photon

Photon has a energy E = h,

According to Einstein, photon energy is utilized in two ways,

electron from the metal surface and it is called work function, and the other one is to

give maximum kinetic energy.

Einstein equation satisfactorily explained all the phenomenon of

photo electric effect observations

1. According to Einstein equation , kinetic energy depends on

frequency and it is independent of intensity Because in

Einstein picture , energy interacts in the form of quanta

2. From the above equation it is clear , in order to have positive

kinetic energy value , frequency of light should always be


3. In this picture , Intensity of light is more means , the number of quanta per

unit time is more , hence more quanta means , more number of photo electrons

will be emitted


Consequently, the greater the intensity the greater should be the energy absorbed by

each electron. In this picture the maximum kinetic energy of photo electrons on the

surface is then expected to increase with intensity.

, no matter what the frequency of radiation (over sufficient time) should

to the electrons, so they exceed the minimum energy needed to

escape from the metal surface. A threshold frequency should not exist.

re should be enough time delay between the light falling on

the emission of electrons from the metallic surface. But it was


9. Write a short note on Einstein Explanation of Photoelectric effect

g to Einstein, Energy interacts with matter in the form of packets of energy

called quanta. Later this quanta is termed as a photon

its energy depends on frequency.

According to Einstein, photon energy is utilized in two ways, one way is to set free the


give maximum kinetic energy.

Einstein equation satisfactorily explained all the phenomenon of

photo electric effect observations.

According to Einstein equation , kinetic energy depends on

frequency and it is independent of intensity Because in

Einstein picture , energy interacts in the form of quanta

From the above equation it is clear , in order to have positive

lue , frequency of light should always be


In this picture , Intensity of light is more means , the number of quanta per



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eater should be the energy absorbed by

photo electrons on the

should be able to

minimum energy needed to

between the light falling on

the metallic surface. But it was


anation of Photoelectric effect?

g to Einstein, Energy interacts with matter in the form of packets of energy

one way is to set free the


In this picture , Intensity of light is more means , the number of quanta per



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4. In Einstein picture, photon interacts with matter. That is an elementary

process. It is always instantaneous whatever the dim of light may be.

Millikan experimented with lot of alkali metals and found a straight line graph

between stopping potential and frequency. Straight line have slope (h/e), as

charge of electron is known, Plancks constant is calculated perfectly from these

graphs.

V0 = (h/e) (0/e)

Thus Millikan successfully Proved Einstein explanation of photo electric effect.

10. What are the properties of photons?

A) Einstein thought that light has got particle like behavior during its interaction with

matter. According to him, light consists of particles associated with definite amount of

energy and momentum. These particles were termed as Photons.

A photon is a packet of energy given by E = h = hc/.

Where h is the Plancks constant, = wavelength of light, = frequency of light

Properties of photons:

1. A photon travels with the speed of light in vacuum

2. A photon has zero rest mass

3. Photons do not have any charge , so they are electrically neutral

4. They are not affected by electric and magnetic fields

5. Kinetic mass of photon is given by m = E/c2 = h/c2 = h/c

6. Magnitude of momentum of photon is given by P = E/c = h/c = h/

11. Define photo electric cell?

A) A device which converts light energy in to electrical energy is known as photo

electric cell or photo cell. They are of mainly three types

1. Photo emissive cell

2. Photo conductive cell

3. Photo voltaic cell

Photo electric cells are used in Automatic fire alarms , Automatic Burglar

Alarm , Scanners , Reproduction of sound in cinema films , to locate flaws or

holes in finished goods etc.,


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12. Explain Debroglie hypothesis?

A) According to Debroglie, Particles like electron, proton and neutron also have

both wave and particle nature. The waves associated with moving particles are

called matter waves and the wavelength is called Debroglie wavelength.

According to quantum theory, energy of photon is given by E = h

According to Einstein mass energy equivalence, the energy of photon is E=

mc2.

mc2 = h

mc2 = hc/

mc = h/ or = h/mc or = h/p

If instead of photon, we have a material particle of mass m moving with a

velocity v , then = h/mv

Conclusions from Debroglie hypothesis :

1. Debroglie wavelength is inversely proportional to the velocity of the

particle, if particle moves faster , wavelength of the wave associated with

it is smaller or vice versa

2. If the particle is at rest, then the Debroglie wavelength is infinite.

3. Debroglie wavelength is inversely proportional to mass of particle, thus

the wavelength of a wave due to heavier particle is smaller than due to a

lighter particle.

4. It is independent of the charge on the material particle.

Note: For our daily life objects, the wavelengths associated with those are very

small that is beyond any measurement. Thats why we cannot detect wave like

properties in our daily lives

13. Explain briefly Davisson and Germer Experiment?

A) Davisson and germer performed an experiment to study the wave nature of

electrons. They used the diffraction effects of electron radiation scattered by crystals.

The main components of the set up are

1. Electron gun: it consists of tungsten filament with barium oxide coating. This

filament acts as an emitter. It is heated by low tension battery to give

thermionic emission. The emitted electrons are accelerated through high


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tension source. A guided cylinder with the hole arranges these electrons in to a

collimated beam.

2. Nickel crystal: The beam of electrons is incident on the Nickel crystal. Atoms of

the crystal scatter the beam in all directions.

3. Detector or collector: It can be moved on circular scale. It is further connected

to a sensitive galvanometer to detect current proportional to the intensity of

electron beam.

By moving the detector on the circular scale at different positions, the intensity of

scattered electron beam is measured for different values of angle of scattering, which

is the angle between the incident and scattered electron beams.

The variation of intensity of the scattered electrons with the angle of scattering is

obtained for different accelerating voltages. For a range of voltages 44V to 68V, it was

noticed that a strong peak appeared in the intensity of the scattered electrons for an

accelerating voltage of 54 V at a scattering angle 50 degrees.

The appearance of peak in a particular direction is due to the constructive interference

of electrons scattered from different layers of the regularly spaced atoms of the

crystals.

The Debroglie wavelength associated with the electrons at voltage 54 V is given by

0.167 nm. From the electron diffraction measurement the wavelength of matter

waves was found to be 0.165 nm.

Thus there is an excellent agreement between the theoretical value and the

experimentally obtained value of Debroglie wavelength. Thus it confirms the wave

nature of matter.

Numerical problems:

Numerical problems will appear from the following areas

1. Photoelectric effect

2. Debroglie waves


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Dual Nature of Radiation and Matter