modern physics| Photons| complete notes| class 12|NEB|

photon

 photon is a packets of light . photon travels in a straight line with speed of light in a vacuum. it has zero rest mass, i.e. photon cannot exist at rest. they are electrically neutral and are not deviated by electric or magnetic field . wavelength of photons changes in different media, so the velocity of photon in different in different media.

EINSTEINS S PHOTOELECTRIC EQUATIONS;STOPPING POTENTIALS

whenever light or electromagnetic radiations such as x rays , ultraviolet rays fall on a metal surface. Some electrons are emitted from the surface. this phenomenon of emission from its metallic surface when radiations of suitable frequency falls on it is called photoelectric effect. These electrons are called photoelectrons.

metals like zinc, cadmium are more sensitive only to ultraviolet light where as alkali metals like sodium, potassium, are sensitive even to visible light.

EFFECT OF INTENSITY OF INCIDENT LIGHT ON THE PHOTOELECTRIC CURRENT

since photo electric  current is directly proportional to the number of photo electrons emitted per second, therefore the number of photoelectrons emitted per second is directly proportional to the intensity of incident light.

EFFECT OF POTENTIAL ON PHOTOELECTRIC CURRENT

when light of suitable frequency falls on the photo sensitive electrode C ,photoelectrons are emitted. these electrons get accelerated towards the electrode A  which is at positive  potentials with respect to the electrode C and constitute the current called photoelectric current. for fixed frequency and intensity of incident light, this photoelectric current increases with the increase in applied positive potentials in plates A. when all the photoelectrons  emitted by electrode C reach the plate A , the photoelectric  current  attains maximum value known as saturation current .this saturations current will not increase with further increase with the further increase in positive potentials A .

Now the potentials of plates A is decreased such that it attains negative potentials with the respect to electrode C. the negative potentials applied to plates A is increased  to certain value . the minimum negative potentials applied to plate A for which photoelectric current becomes zero is called cut off potentials or stopping potentials.  

EFFECT OF FREQUENCY ON INCIDENT LIGHT ON STOPPING POTENTAIL

let the intensity of incident light is kept constant and frequency is changed. in each case the saturations current is exactly same. Now for a given frequency f1 of the incident light, the positive potentials at plate A is decreased  to zero.it is found that the photo electric current decreases, Now the plate A is given to the negative which is increased till the photoelectric current become zero.let this value of negative potentials be v0 .if this experiments is repeated with the incident light of frequency f2>f1, it is found  that the stopping potentials is higher than v01 .let it be v02 as shown in fig. thus we found that the value of stopping potentials depend

upon the frequency of the incidents light. stopping potentials is directly proportional to the light incident.

THRESHOLD FREQUENCY

when a graph is plotted between  the frequency of incident light and the stopping potential, it is found as a straight line as shown in the figure above .it shows that there is a minimum value of frequency f0 of the incident light below which photoelectric emission is not possible. This frequency is known as threshold frequency or cutoff frequency f0. this value of threshold frequency depend upon the nature of the substance emitting the photoelectrons.

laws of photoelectric emission

• The photoelectric effect depends upon the frequency of the radiation it doesn't depend upon the frequency of the radiations.
• the rate of emission is directly proportional to the intensity of the light .
• the velocity and energy of photoelectrons depends upon the frequency of the radiations but does not depends upon the intensity.
• the stopping potentials also independent to the intensity of radiations but it directly depends to the frequency of the radiations.
• there is an instantaneous emission of photoelectric within the limits of experienced accuracy.

some important terms of  photoelectric effect.

1. threshold frequency; the minimum value of frequency of incident radiations which is sufficient to just eject the electrons from the metal surface with zero velocity.
2. threshold wavelength: the wavelength corresponding to the threshold frequency is called threshold wavelength .
3. work function; the minimum amount of potentials required to move the electrons from metal surface is called wave functions.

EINSTEINS S PHOTOELECTRIC EQUATIONs

it states that when the radiations having suitable frequency fall on the metal surface then some energy is used to eject the electron and remains energy is used to move a electron.

mathematically,

E=workfunction+max kinetic energy

APPLICATIONS OF PHOTOELECTRIC CELLS

1. They are used to switch on and off the automatics switches of the streets light.
2. they are used to operates controls in electrical devices such as televisions and mobile phones.
3. they are also used in reproductions sound cinematography. 
4. it is used in burglar alarm and fire alarm.
5. it is used in the industry to locate minor flaws or holes in metal sheets.
6. they are used in controlling temperatures of furnaces and chemicals reactions.
7. photelectric cells are used for counting operations.
8. they are used to measures the temperature of starts and to study spectrums of heavily bodies  and in scientific research.

numerical problems

1. A photons of green light has a wavelength of 520nm. find the photons frequency, magnitude of momentum and energy. Express the energy both in joules and electrons volt. 🖋
2. an excited nucleus emits a gamma rays photon with an energy of 5.56mev.find

• what is the photon frequency?
• what is the wavelength of the photon? 🖋

3.The predominant wavelength emitted by an ultraviolet lamp is 248nm.if the total power imitate this wavelength is 20w. how many photons are emitted from the surface?🖋

4.A clean Nickle surface is exposed to the light of wavelength 235nm.what is the maximum speed of the photoelectrons emitted from this surface. 🖋

5.light of frequency 50000hz liberates electrons with the energy 2.31*100j from a certain metallic surfaces. what is the wavelength of ultra  light which is liberated from the energy 4mev from the same surface.

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