Question:
Suppose that the potential energy between an electron and a proton at a distance r is given by -ke2/3r2 . Use Bohr theory to obtain energy level of such a hypothetical hydrogen atom.
Solution:
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Showing posts with label Modern Physics. Show all posts
Showing posts with label Modern Physics. Show all posts
Sunday, 10 April 2011
Saturday, 2 April 2011
Problem on Bohr atom model
Question:
A particle of charge equal to that of electron and mass 208 times the mass of electron (-meson)moves in a circular orbit around a nucleus of charge +3e (assume mass of nucleus to be infinite). Assuming that the Bohr atom model is applicable to this system
(a) derive an expression for the radius of the nth Bohr orbit
(b) find the value of n for which the radius of the orbit is approximately the same as that of the first Bohr orbit for the hydrogen atom
(c) find the wavelength of the radiation emitted when -meson jumps from the third orbit to the first orbit (Rydberg constant = 1.097 x 107 m-1)
A particle of charge equal to that of electron and mass 208 times the mass of electron (-meson)moves in a circular orbit around a nucleus of charge +3e (assume mass of nucleus to be infinite). Assuming that the Bohr atom model is applicable to this system
(a) derive an expression for the radius of the nth Bohr orbit
(b) find the value of n for which the radius of the orbit is approximately the same as that of the first Bohr orbit for the hydrogen atom
(c) find the wavelength of the radiation emitted when -meson jumps from the third orbit to the first orbit (Rydberg constant = 1.097 x 107 m-1)
Solution:
For solution click this link
Friday, 11 March 2011
Nuclear Forces
We already know about gravitational and electrostatic forces now a question arises that does these forces still hold when we talk about nuclear dimensions as gravitational force between two protons is far too less to hold them and electrostatic force between two protons always a force repulsive in nature . So what hold protons together to form a nucleus. Answer is Nuclear forces which are attractive in nature and strong enough to overcome the electrostatic force of repulsion between two protons. So it is the nuclear forces which holds neutrons and protons (nucleons) together to form a nucleus. Nucleus forces are complex in nature and difficult to understand. Some of the properties of nuclear forces are given below.
(1) Nuclear forces are strong attractive forces :- These are strongest forces and are about 100 times stronger then electrostatic forces and about 1038 times stronger then gravitational forces. These are the forces which holds nucleons in a nucleus.
(2) Nuclear forces are short range forces :- The nuclear forces are most effective up to a distance of the order of 10-15m or less. These forces decreases rapidly as the separation distance between nucleons increases and becomes negligible when separation between nucleons is of the order of 10 fermi . The effective range up to which nuclear forces are effective is known as nuclear range.
(3) Nuclear forces are the charge independent :- Nuclear forces acting between two protons , two neutrons and between a proton and neutron have same strength and the nature of force is attractive.
(4) Nuclear forces are not central forces :- These forces do not obey inverse square law. The magnitude of nuclear force depends on the direction of spin of nucleons.
(5) Nuclear forces are saturated forces :- A nucleon experiences the force only due to it nearest neighbouring nucleons.
(1) Nuclear forces are strong attractive forces :- These are strongest forces and are about 100 times stronger then electrostatic forces and about 1038 times stronger then gravitational forces. These are the forces which holds nucleons in a nucleus.
(2) Nuclear forces are short range forces :- The nuclear forces are most effective up to a distance of the order of 10-15m or less. These forces decreases rapidly as the separation distance between nucleons increases and becomes negligible when separation between nucleons is of the order of 10 fermi . The effective range up to which nuclear forces are effective is known as nuclear range.
(3) Nuclear forces are the charge independent :- Nuclear forces acting between two protons , two neutrons and between a proton and neutron have same strength and the nature of force is attractive.
(4) Nuclear forces are not central forces :- These forces do not obey inverse square law. The magnitude of nuclear force depends on the direction of spin of nucleons.
(5) Nuclear forces are saturated forces :- A nucleon experiences the force only due to it nearest neighbouring nucleons.
Thursday, 24 February 2011
Thursday, 17 February 2011
Rutherford's nucleus model
According to this model most of the mass of atom and all its positive charge are concentrated in tiny nucleus, and electrons in the atom revolve around it. This model of atomic structure emerged from Geiger - Marsden experiment in 1911 in which a collimated beam of 5.5 MeV alpha particles from bismuth was allowed to fall on 2.1 x 10-7 mm thin gold foil. The scattered alpha particles produced scintillations on ZnS screen, which were counted at different angles (θ) from the direction of the beam.. though most of the alphs particles suffer negligible deviation , some (about one in 104) suffered a large change in direction (θ>90) The last observation gave crutial clue to the nuclear model.
Rutherford's calculation used the inverse square law of repulsive force between alpha particle and the gold nucleus. Multiple scattering was ignored. The scattering angle θ of alpha particle is related to impact parameter b by the relation
b=[Ze2 cot(θ/2)]/[4πε0(mv2/2)]
where impact parameter is defined as the perpandicular distance of the initial velocity vector of the alpha particle from the centre of the nucleus. The observed number of scattered alpha particles at different angles agreed with Rutherford's calculation based on the nuclear model of atom.
Classically Rutherford's model of atom is unstable because an orbiting electron accelerates continously and must loose its energy as EM radiation. The orbit should shrink spiraly into the nucleus within 10-8 sec and gives out continous spectrum of radiation. But we now that hydrogen atom is stable and has a characterstic line spectrum . This difficulty of unstable and shrinking orbits was taken care in Bohr's model of hydrogen atom and would be discussed later.
Rutherford's calculation used the inverse square law of repulsive force between alpha particle and the gold nucleus. Multiple scattering was ignored. The scattering angle θ of alpha particle is related to impact parameter b by the relation
b=[Ze2 cot(θ/2)]/[4πε0(mv2/2)]
where impact parameter is defined as the perpandicular distance of the initial velocity vector of the alpha particle from the centre of the nucleus. The observed number of scattered alpha particles at different angles agreed with Rutherford's calculation based on the nuclear model of atom.
Classically Rutherford's model of atom is unstable because an orbiting electron accelerates continously and must loose its energy as EM radiation. The orbit should shrink spiraly into the nucleus within 10-8 sec and gives out continous spectrum of radiation. But we now that hydrogen atom is stable and has a characterstic line spectrum . This difficulty of unstable and shrinking orbits was taken care in Bohr's model of hydrogen atom and would be discussed later.
Tuesday, 15 February 2011
X-Rays FACT FILE
1. X-Rays are emitted from metal when metal surface is hit by high energy electrons. The electrons penetrate close to the nucleus and displace electrons around the nucleus. The enerfy change is equal to hf where f is the frequency and is very high for radiation emitted.
2. X-Rays are EM waves of very high frequency and short wavelength near about a 100 times shorter then the wavelength of visible light.
3. Moseley’s law : Moseley measured the frequencies of characteristic X-rays from large number of elements and plotted the square root of th frequency against its position number in the periodic table.
His observations are mathematically expressed as
√(v) = a(Z-b)
v = the frequency of characteristic X-rays from the elements\
Z = atomic number
a, b are constants
Bragg’s law :Atomic structure of crystals can be studied by X Ray analysis and was started in 1914 by Sir William Bragg with notable achievements.
Bragg's law is :- 2d sin θ = n λ
d = interplanar spacing of the crystal on which X-rays are incident
θ = is the incident angle at which X-rays are strongly reflected.
n = 1,2,3 …
λ = wave length of X-rays
Application of Bragg’s law : By using a monochromatic X-ray beam (having a single wave length) and noting the angles of strong reflection, the interplanar spacing d and several information about the structure of the crystals can be obtained.
4. x-rays do not contain charged particles. hence they are not deflected by electric or magnetic field.
5. They effect a photographic plate. The effect is stronger than light.
6. When incident on certain materials barium platinocyanide, X rays cause fluorescence.
7. When passed through a gas, X rays ionize the molecules of the gas.
2. X-Rays are EM waves of very high frequency and short wavelength near about a 100 times shorter then the wavelength of visible light.
3. Moseley’s law : Moseley measured the frequencies of characteristic X-rays from large number of elements and plotted the square root of th frequency against its position number in the periodic table.
His observations are mathematically expressed as
√(v) = a(Z-b)
v = the frequency of characteristic X-rays from the elements\
Z = atomic number
a, b are constants
Bragg’s law :Atomic structure of crystals can be studied by X Ray analysis and was started in 1914 by Sir William Bragg with notable achievements.
Bragg's law is :- 2d sin θ = n λ
d = interplanar spacing of the crystal on which X-rays are incident
θ = is the incident angle at which X-rays are strongly reflected.
n = 1,2,3 …
λ = wave length of X-rays
Application of Bragg’s law : By using a monochromatic X-ray beam (having a single wave length) and noting the angles of strong reflection, the interplanar spacing d and several information about the structure of the crystals can be obtained.
4. x-rays do not contain charged particles. hence they are not deflected by electric or magnetic field.
5. They effect a photographic plate. The effect is stronger than light.
6. When incident on certain materials barium platinocyanide, X rays cause fluorescence.
7. When passed through a gas, X rays ionize the molecules of the gas.
Thursday, 10 February 2011
Light : Waves or particles
When we think of light a question comes to our mind whether it light is a wave or a particle. This discussion is very interesting and has got a long history. Newton the great physicist tried to understand travel of light in straight line assuming that a luminous body emits very minute and weightless particles called corpuscles travelling through empty space in straight line in all directions with the speed of light and carry kinetic energy with them. Thus energy is carried by stream of particles travelling with a finite velocity , this is the basic principle behind what we call the Corpuscular theory proposed by Sir Issac Newton. This Corpuscular theory of light can fairly explain the phenomenon of reflection , refraction and rectilinear propagation of light but failed to explain phenomenon of interference , diffraction and polarization of light.
A new theory of propagation of light was suggested by Dutch physicist Christian Huygens in 1678 in which he suggested that light may be a wave phenomenon produced by mechanical vibrations of an all pervading hypothetical homogeneous medium called ether just like those in liquids and solid. This medium was supposed to be mass less with extremely high elasticity and very low density. In this theory there is a transfer of energy by wave motion without actual travelling of matter. At first wave theory of light was not accepted primarily because of Newton's authority and also because light could travel through vacuum and waves require a medium to propagate from one place to another. Wave theory of light first begin to gain acceptance when double slit experiment of Thomas Young in 1801 firmly established that light is indeed a wave phenomenon. After this double slit interference experiment many experiments were carried out by scientists involving interference and diffraction of light which could only be explained by assuming wave model of light.
Later on in nineteenth century Maxwell put forward his electromagnetic theory and predicted the existence of electromagnetic waves and calculated the speed of EM waves in free space and fount that this value was very close to the measured value of speed of light in vacuum. He then suggested that light must be an EM wave associated with changing electric and magnetic fields which results the propagation of light or EM waves even in the vacuum. So this way mo material medium is required for the propagation of light wave travelling from one place to another. This fact established that light is a wave phenomenon.
But this is not the end of the story Hertz first observed the phenomenon of photoelectric effect in1800 according to which when light falls on metal surface , electrons are emitted from the metal surface and the kinetic energy of the electrons does not depend on the intensity of light used. This phenomenon was latter explained successfully by another great physicist Albert Einstein in 1905 by assuming light as photons the quanta of light. His theory again gave rise to the old discussion whether light is a wave or particle. Later on well established particles like electrons also shows diffraction phenomenon under suitable conditions and such effects csn be studied under wave particle duality beyond the scope of this article.
A new theory of propagation of light was suggested by Dutch physicist Christian Huygens in 1678 in which he suggested that light may be a wave phenomenon produced by mechanical vibrations of an all pervading hypothetical homogeneous medium called ether just like those in liquids and solid. This medium was supposed to be mass less with extremely high elasticity and very low density. In this theory there is a transfer of energy by wave motion without actual travelling of matter. At first wave theory of light was not accepted primarily because of Newton's authority and also because light could travel through vacuum and waves require a medium to propagate from one place to another. Wave theory of light first begin to gain acceptance when double slit experiment of Thomas Young in 1801 firmly established that light is indeed a wave phenomenon. After this double slit interference experiment many experiments were carried out by scientists involving interference and diffraction of light which could only be explained by assuming wave model of light.
Later on in nineteenth century Maxwell put forward his electromagnetic theory and predicted the existence of electromagnetic waves and calculated the speed of EM waves in free space and fount that this value was very close to the measured value of speed of light in vacuum. He then suggested that light must be an EM wave associated with changing electric and magnetic fields which results the propagation of light or EM waves even in the vacuum. So this way mo material medium is required for the propagation of light wave travelling from one place to another. This fact established that light is a wave phenomenon.
But this is not the end of the story Hertz first observed the phenomenon of photoelectric effect in1800 according to which when light falls on metal surface , electrons are emitted from the metal surface and the kinetic energy of the electrons does not depend on the intensity of light used. This phenomenon was latter explained successfully by another great physicist Albert Einstein in 1905 by assuming light as photons the quanta of light. His theory again gave rise to the old discussion whether light is a wave or particle. Later on well established particles like electrons also shows diffraction phenomenon under suitable conditions and such effects csn be studied under wave particle duality beyond the scope of this article.
Sunday, 21 November 2010
CSIR NET physics: Ferromagnetism (in short) Part 1
For reading ferromagnetism for higher level visit the link given below
CSIR NET physics: Ferromagnetism (in short) Part 1: "A ferromagnetic material has a spontaneous magnetic moment- magnetic moment even in zero applied magnetic field this means that electron s..."
CSIR NET physics : Ferromagnetism (in short) part 2: "Nature of Ferromagnetic carriers : Entire magnetization must be essentially associated with electron spin, and not at all with the orbital motion of electrons....."
Hope you like it
physics expert
CSIR NET physics: Ferromagnetism (in short) Part 1: "A ferromagnetic material has a spontaneous magnetic moment- magnetic moment even in zero applied magnetic field this means that electron s..."
CSIR NET physics : Ferromagnetism (in short) part 2: "Nature of Ferromagnetic carriers : Entire magnetization must be essentially associated with electron spin, and not at all with the orbital motion of electrons....."
Hope you like it
physics expert
Wednesday, 17 November 2010
Tuesday, 14 September 2010
Crystals
Many materials like carbon can exist in various forms for example it can exist as diamond in solid form and graphite is also one form of carbon and clearly dimond and graphite have different mechanical, thermal, optical and electrical properties. These properties of different forms of carbon can be understood in terms of carbon atoms in the solid structure.
The atoms having no regular arrangement of atoms in their structure are called amorphous solids and those having regular or orderly arrangement of atoms in their structure are called crystalline solids. Graphite is an amorphous solid and diamind is a crystalline one.
Crystalline solids or crystals have periodic arrangements constructed by infinite repetion of identical structural units in space. In simple crystals structural unit is a single atom but it could be group of atoms or molecules attached to every lattice point. Structure of all crystals is described in terms of lattice.
One of simple type of crystal structures is Sodium chloride is a crystal structure having face centered cubic crystal lattice. Its basis consists of one Na and one Cl atom separated by half the body digonal of a unit cube. Each atom in sodium chloride cryatal has six nearest atoms of opposite kind. Space lattice of diamond is a face centered cubic crystal. The conventional unit cube of a diamond crystal contains 8 atoms and the basis of diamond contains only one atom that is carbon atom. Thus each point of fcc lattice have two identical atoms attached as its primitive basis.
This post is purely an informational one and is beyond your course you intend to study for your board and competetive exams.
The atoms having no regular arrangement of atoms in their structure are called amorphous solids and those having regular or orderly arrangement of atoms in their structure are called crystalline solids. Graphite is an amorphous solid and diamind is a crystalline one.
Crystalline solids or crystals have periodic arrangements constructed by infinite repetion of identical structural units in space. In simple crystals structural unit is a single atom but it could be group of atoms or molecules attached to every lattice point. Structure of all crystals is described in terms of lattice.
One of simple type of crystal structures is Sodium chloride is a crystal structure having face centered cubic crystal lattice. Its basis consists of one Na and one Cl atom separated by half the body digonal of a unit cube. Each atom in sodium chloride cryatal has six nearest atoms of opposite kind. Space lattice of diamond is a face centered cubic crystal. The conventional unit cube of a diamond crystal contains 8 atoms and the basis of diamond contains only one atom that is carbon atom. Thus each point of fcc lattice have two identical atoms attached as its primitive basis.
This post is purely an informational one and is beyond your course you intend to study for your board and competetive exams.
Monday, 6 September 2010
Superconductors and Superconductivity
Phenomenon of superconductivity was first observed by Kamerlingh Onnes in Leiden in 1911 ,3 years after he first liquified helium gas. He observed that electrical resistivity of such as mercury, tin, lead completely disappear i.e., suddenly dropped to zero in a small range of temperature at a critical temperature TC which is the characterstics of the material. This heppens when the specimen is cooled down to sufficiently low temperature about few degrees of Kelvin. At critical temperature TC specimen undergoes a phase transition from normal electrical resistivity state to superconducting state. Magnetic properties exhibited by superconductors are as interesting as its electrical properties. Now imagine what happened when a specimen known to exhibit superconductivity is placed in a magnetic field and s then cooled through the transition temperature for superconductivity. In this case magnetic field originally present in the specimen is ejected out from the specimen and this is nothing but the Meissner Effect. Information i had given here in this page is nothing but a mere definition of superconductivity phenomenon. It is a very vast topic and has a lot in it. For more information on superconductors you can visit
Tuesday, 29 June 2010
Atomic nucleus
(A) Introduction
1. Atomic nucleus exists at the center of the atom. Nucleus of an atom consists of entire positive charge of the atom and almost all of its mass.
2. Nuclei is made up of protons and neutrons out of which protons are positively charged particles and neutrons does not posess any charge.
3. Neutrons and protons togather are known as neutrons.
(B) Protons
1. Protons are positively charged particles.
2. Magnitude of charge on a proton is +1.6x10-19 C.
3. Mass of a proton is 1841 times greater then that of an electron.
4. Total number of protons in a atomic nucleus (Z-atomic number) is equal to the number of electrons in the neutral atom.
(c) Neutron
1. Neutron is a chargeless particle.
2. Mass of a neutron is aproximately equal to the mass of a proton.
3. The number of protons plus the number of neutrons in the nucleus is known as mass number of the nucleus.
(D) Definations
1. Nuclide:- A single nuclear species (single value of both Z and N ) is called nuclide and it is represented as ZXA.
2. Isotopes:- Nuclides having same number of protons but different number of neutrons.
3. Isobars:- Nuclides having same number of nucleons but different number of protons.
4. Isotones:- Nuclides having same number of neutrons but different number of protons.
(E) Properties of nucleus
1. The radius of nucleus is of the order of 10-15.
2. Radius of nuclei is represented by the relation
R=R0A1/3
where R0=1.2x10-15 is a constant same for all the nuclei.
3. Volume of the nuclei is proportional to A.
4. Density of nuclei is very high about 1012 times greater then that of lead.
5. Nucleus also posess angular momentum and magnetic moment which arises as a result of continous motion of neutrons and protons inside the atomic nucleus.
Complete notes of various chapters in physics are available at website also visit the website for one year free online learning program in physics OLP. This OLP would prove helpful for those preparing for various competetive exams like IITJEE, AIEEE, PMT, CBSE and also for board examinations.
1. Atomic nucleus exists at the center of the atom. Nucleus of an atom consists of entire positive charge of the atom and almost all of its mass.
2. Nuclei is made up of protons and neutrons out of which protons are positively charged particles and neutrons does not posess any charge.
3. Neutrons and protons togather are known as neutrons.
(B) Protons
1. Protons are positively charged particles.
2. Magnitude of charge on a proton is +1.6x10-19 C.
3. Mass of a proton is 1841 times greater then that of an electron.
4. Total number of protons in a atomic nucleus (Z-atomic number) is equal to the number of electrons in the neutral atom.
(c) Neutron
1. Neutron is a chargeless particle.
2. Mass of a neutron is aproximately equal to the mass of a proton.
3. The number of protons plus the number of neutrons in the nucleus is known as mass number of the nucleus.
(D) Definations
1. Nuclide:- A single nuclear species (single value of both Z and N ) is called nuclide and it is represented as ZXA.
2. Isotopes:- Nuclides having same number of protons but different number of neutrons.
3. Isobars:- Nuclides having same number of nucleons but different number of protons.
4. Isotones:- Nuclides having same number of neutrons but different number of protons.
(E) Properties of nucleus
1. The radius of nucleus is of the order of 10-15.
2. Radius of nuclei is represented by the relation
R=R0A1/3
where R0=1.2x10-15 is a constant same for all the nuclei.
3. Volume of the nuclei is proportional to A.
4. Density of nuclei is very high about 1012 times greater then that of lead.
5. Nucleus also posess angular momentum and magnetic moment which arises as a result of continous motion of neutrons and protons inside the atomic nucleus.
Complete notes of various chapters in physics are available at website also visit the website for one year free online learning program in physics OLP. This OLP would prove helpful for those preparing for various competetive exams like IITJEE, AIEEE, PMT, CBSE and also for board examinations.
Thursday, 21 August 2008
Modern Physics Test Series
(A) Multple choice questions
1. Photoelectric emission takes place when
(a)electrons falls on photosensitive surface
(b)alpha particles falls on photosensitive surface
(c)light rays falls on photosensitive surface
(d)neutrons falls on photosensitive surface
2. The velocity of most energetic photo electrons emitted from metal surface depends on
(a) time for which radiation fall on metal surface
(b) the frequency of incident light
(c) the intensity of incident light
(d) none of the above
3. The work funcion of a metal is w and λ is the wavelength of incident radiation. there would be no emission of photoelectrons when
(a) λ > hc/w
(b) λ < hc/w
(c) λ = hc/w
(d) λ ≤ hc/w
4. The fact that was not inferred from the Rutherford model of the atom was that the
(a) nucleus occupied a small space inside the atom
(b) nucleus consisted of neutrons and protons
(c) electrons were outside the nucleus of the atom
(d) nucleus was dense in nature
5. A graph showing the energy of the photoelectron as a fuction of the frequency of the radiation for a given emitting surface is best represented as
(a)
(b)
(c)
(d)
(B) Matrix match type question
6. Match the items mentioned with those given in group (P) with those given in group (Q)
(P)
(a) The concept of nuclear atom was
established from the experiment on the
(b) Wave aspect of matter was confirmed by
(c) Bohr theory was successful in explaining
(d) Photons may cause
(Q)
(1) emission of electrons from the metal surface
(2) scattering of alpha particles by a gold foil
(3) diffraction of electrons by a crystal
(4) bright line spectra of hydrogen atom
(5) scattering of X-rays by electrons
Answers
1. (c)
2. (b)
3. (a)
4. (c)
5. (a)
6. a-2
b-3
c-4
d-1
1. Photoelectric emission takes place when
(a)electrons falls on photosensitive surface
(b)alpha particles falls on photosensitive surface
(c)light rays falls on photosensitive surface
(d)neutrons falls on photosensitive surface
2. The velocity of most energetic photo electrons emitted from metal surface depends on
(a) time for which radiation fall on metal surface
(b) the frequency of incident light
(c) the intensity of incident light
(d) none of the above
3. The work funcion of a metal is w and λ is the wavelength of incident radiation. there would be no emission of photoelectrons when
(a) λ > hc/w
(b) λ < hc/w
(c) λ = hc/w
(d) λ ≤ hc/w
4. The fact that was not inferred from the Rutherford model of the atom was that the
(a) nucleus occupied a small space inside the atom
(b) nucleus consisted of neutrons and protons
(c) electrons were outside the nucleus of the atom
(d) nucleus was dense in nature
5. A graph showing the energy of the photoelectron as a fuction of the frequency of the radiation for a given emitting surface is best represented as
(a)
(b)
(c)
(d)
(B) Matrix match type question
6. Match the items mentioned with those given in group (P) with those given in group (Q)
(P)
(a) The concept of nuclear atom was
established from the experiment on the
(b) Wave aspect of matter was confirmed by
(c) Bohr theory was successful in explaining
(d) Photons may cause
(Q)
(1) emission of electrons from the metal surface
(2) scattering of alpha particles by a gold foil
(3) diffraction of electrons by a crystal
(4) bright line spectra of hydrogen atom
(5) scattering of X-rays by electrons
Answers
1. (c)
2. (b)
3. (a)
4. (c)
5. (a)
6. a-2
b-3
c-4
d-1
Friday, 20 June 2008
PARTICAL PROPERTIES OF WAVES
Photoelectric effect
-Photoelectric effect is the emission of electrons from metal surface when they absorb energy from an electromagnetic wave.
-These electrons emitted from metal surface are known as photo electrons.
(a) Observed features:
-For a given metal or photosenstive surface there is a minimum frequency ν0 of incident radiation , below which there is no emission of photo electrons and this frequency is called threshold frequency of that metal surface.
-Electronic emission increases with the intensity of radiation falling on the metal surface , since more energy is available to the release of electrons but maximum kinetic energy KEmax (=eV where V is the cut off voltage) is independent of the intensity of light.
-There is no time delay between the arrival of light on the metal surface and the emission of electrons.
-The maximum kinetic energy of photoelectrons and frequency of incident light are related linearly as
KEmax=eV ∝ (ν-ν0)
(b) Theoretical explaination:
-Classical theory which assumes light as an EM wave fails to explain photoelectric effect.
-Einstein first gave correct explaination of photoelectric effect using Plank's idea of energy quantization.
-Einstein in his theory considered that radiation of frequency ν consists of a stream of discrete quanta (photons) each of energy hν , where h is the plank's constant. The photons moves through space with the speed of light.
-When a quanta of energy hν is incident on a metal surface , the entire energy of the photon is absorbed by a single electron without any time lag.
-The minimum amount of work or energy necessary to take a free electron just out of metal against attractive forces of surrounding positive ions is called work function of the metal and is denoted by φ0.
-Appliying principle of conservation of energy to the absorption of photon by an electron in the metal surface we get,
hν=KEmax+φ0
writing φ0=hν0 , we get
KEmax=h(ν-ν0)
-thus maximum kinetic energy is same as observed KEmax with the proportionality constant equal to Plank's constant.
-Photoelectric effect is the emission of electrons from metal surface when they absorb energy from an electromagnetic wave.
-These electrons emitted from metal surface are known as photo electrons.
(a) Observed features:
-For a given metal or photosenstive surface there is a minimum frequency ν0 of incident radiation , below which there is no emission of photo electrons and this frequency is called threshold frequency of that metal surface.
-Electronic emission increases with the intensity of radiation falling on the metal surface , since more energy is available to the release of electrons but maximum kinetic energy KEmax (=eV where V is the cut off voltage) is independent of the intensity of light.
-There is no time delay between the arrival of light on the metal surface and the emission of electrons.
-The maximum kinetic energy of photoelectrons and frequency of incident light are related linearly as
KEmax=eV ∝ (ν-ν0)
(b) Theoretical explaination:
-Classical theory which assumes light as an EM wave fails to explain photoelectric effect.
-Einstein first gave correct explaination of photoelectric effect using Plank's idea of energy quantization.
-Einstein in his theory considered that radiation of frequency ν consists of a stream of discrete quanta (photons) each of energy hν , where h is the plank's constant. The photons moves through space with the speed of light.
-When a quanta of energy hν is incident on a metal surface , the entire energy of the photon is absorbed by a single electron without any time lag.
-The minimum amount of work or energy necessary to take a free electron just out of metal against attractive forces of surrounding positive ions is called work function of the metal and is denoted by φ0.
-Appliying principle of conservation of energy to the absorption of photon by an electron in the metal surface we get,
hν=KEmax+φ0
writing φ0=hν0 , we get
KEmax=h(ν-ν0)
-thus maximum kinetic energy is same as observed KEmax with the proportionality constant equal to Plank's constant.
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