CBSE 12th Standard Physics Subject Dual Nature of Radiation and Matter Value Based Questions 5 Mark Questions With Solution 2021
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CBSE 12th Standard Physics Subject Dual Nature of Radiation and Matter Value Based Questions 5 Mark Questions With Solution 2021
12th Standard CBSE
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Reg.No. :
Physics
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Work function of a metal is the minimum amount of energy required by an electron to just escape from the metal surface without any kinetic energy. The expression for work function of metal is
\({ \phi }_{ 0 }={ hv }_{ 0 }=hc/{ \lambda }_{ 0 }\)
where h is Plank's constant;v0 is the threshold wavelength and c is the velocity of light in vacuum.
(i) Why different metals has different work function?
(ii) What is the threshold wavelength of the incident radiation for a metal surface whose work function is 1.2 eV. Given \(h=6.63\times { 10 }^{ -34 }Js;\)\(1eV=1.6\times { 10 }^{ -19 }J\)
(iii) What do you learn from this study?(a) -
Radiation has dual nature,i.e., it possesses the properties of both; wave and particle.This prompted de-Broglie to predict dual nature of moving material particles.Thus waves are associated with moving material particles which are called matter waves. The wavelength of matter wave is given by \(\lambda =\frac { h }{ mv } \), where m is the mass, v is the speed of the particle and h is Plank's constant. Read the above paragraph and answer the following questions;
(i) How was the wave nature of the electron established?
(ii) What are the de-Broglie wavelength associated with a particle (i) at rest (ii) moving with infinite speed?
(iii) What are the basic values displayed with this study?(a) -
According to Plank's constant quantum theory of light, every source of radiation emits photons(i.e., packets of energy) which travel in all directions with the same speed (i.e., speed of light). Each photon is of energy \(E=hv=\frac { hc }{ \lambda } \) where h is plank's constant, v is the frequency and \(\lambda \) is wavelength of radiation emitted. The photons emitted from different sources of radiation are different.
Read the above passage and answer the following questions:
(i) On what factors does the number of photons emitted per second from a source of radiation depend?
(ii) Why are the high energy photons not visible to eye?
(iii) Which basic values do you learn from this study?(a) -
"Know your face beauty through comp[lexion meter" was one of the stall on science exhibition. A student interested to know his/her face beauty was made to stand on a platform and light from a lamp was made to fall on his/her face. The reading of complexion meter indicated the face beauty of the student which might be very fair, fair,semi-fair, semidark dark etc.
(i) What is the basic concept used in the working of complexion meter?
(ii) How is the face beauty recorded by face complexation meter?
(iii) What basic values do you learn from the above study?(a) -
The photoelectric effect is a phenomenon of emission of electrons from the surface of a metal when the light of suitable frequency falls on it.
If light of frequency v falls on a photosensitive surface of work function increases or work function then the maximum kinetic energy of photoelectric emitted is given by Einstein's photoelectric equation.
\({ (KE) }_{ max }=\frac { 1 }{ 2 } { { mv }^{ 2 } }_{ max }=hv-{ { \phi }_{ 0 } }\)
The value \({ (KE) }_{ max }\) will increase if the energy of the incident light(hv) increases or work function \({ { \phi }_{ 0 } }\) is decreased.
Read above passage and answer the following questions:
(i) Why can visible light not eject photoelectrons from every metal surface?
(ii) Light of frequency \(7.21\times { 10 }^{ 14 }Hz\) is incident on a metal surface.Electrons with a maximum speed of \(6.0\times { 10 }^{ 5 }{ ms }^{ -1 }\) are ejected from the surface. What is the threshold frequency for photo emission of electrons?\(h=6.63\times { 10 }^{ -34 }Js;\ { m }_{ e }=9.1\times { 10 }^{ -31 }kg\)
(iii) What do you learn basically from the above study?(a)
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CBSE 12th Standard Physics Subject Dual Nature of Radiation and Matter Value Based Questions 5 Mark Questions With Solution 2021 Answer Keys
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Work function of metal depends upon the amount of restraining forces acting on the electrons to be ejected.In different metals, the amount of restraining forces on the electrons to be ejected is different.Hence, work function for different metals is different.
Here,
\({ \phi }_{ 0 }=1.2eV=1.2\times 1.6\times { 10 }^{ -19 }J\)
\( As \ { \phi }_{ 0 }=\frac { hc }{ { \lambda }_{ 0 } } \ or \ { \lambda }_{ 0 }=\frac { hc }{ { \phi }_{ 0 } } =\frac { \left( 6.63\times { 10 }^{ -34 } \right) \times \left( 3\times { 10 }^{ 8 } \right) }{ 1.2\times 1.6\times { 10 }^{ -19 } } =1037\times { 10 }^{ -19 }m=1037nm\)
(iii) Work function of metal required to the minimum energy required by an electron to escape from the metal. In day-to-day life, we find that we can purchase something only when we have money, which is at least equal to or more than that price. Thus to purchase comforts of life, we have to work hard and earn money by fair means. -
(i) Davisson and Germer observed diffraction patterns of slow moving electrons. And G.P. Thomson observed a diffraction pattern of fast moving electrons. As diffraction is essentially a wave phenomenon, therefore, it was concluded that wave must be associated with moving electrons.
(ii) (a) At rest, v = 0, \(=\frac { h }{ mv } =\frac { h }{ m\times 0 } =\infty \)
(b) Particle moving with infinite speed, v = \(\infty \)
\(\lambda =\frac { h }{ mv } =\frac { h }{ m\times \infty } \)
(iii) The dual nature of moving material particles reveals in a way the nature of Almighty God. He is in a visible form (i.e., Sakar) like a visible particle and also without any form (i.e., Nirakar) like a wave.It depends on us how we realize him. -
(i) The number of photons emitted per second from a source of radiation on its temperature, nature and area of radiating surface.
(ii) The ratina of our eye is sensitive to visible light whose frequency range is \(4\times { 10 }^{ 14 }Hz\ to\ 8\times { 10 }^{ 14 }Hz\) High energy photons are frequency much higher than \(8\times { 10 }^{ 14 }Hz\). They do not produce any sensation on our ratina.Hence, they cannot be seen by our eyes.
(iii) We have learnt that every source emits radiations (i.e., photons) The different source emits radiations of different energies, which have different effects on the exposed bodies. The radiations of holy persons cultivate in us the good qualities and radiations of criminals develop bad qualities.Therefore, we must live in a company of good people for better and peaceful life. -
(i) The basic concept used in the working of complexation meter is a photoelectric effect.
(ii) The reading of complexation meter depends on the current generated from the light reflected from the face. Fairer the colour more is the light reflected and vice versa.
(iii) Complexation meters judge your beauty only. The inner beauty of a person is judged by the virtues he or she posseses. In my opinion, inner beauty is much more valuable than the face beauty. -
(i) This is because the energy of visible photons is less than work function of most of the metals.
(ii) \(\frac { 1 }{ 2 } { { mv }^{ 2 } }_{ max }=hv-{ { \phi }_{ 0 }=hv-{ hv }_{ 0 } }\)
\({ v }_{ 0 }=v-\frac { { { mv }^{ 2 } }_{ max } }{ 2h } =7.21\times { 10 }^{ 14 }-\frac { \left( 9.1\times { 10 }^{ -31 }kg \right) \times { \left( 6\times { 10 }^{ 5 } \right) }^{ 2 } }{ 2\times \left( 6.63\times { 10 }^{ -34 } \right) } =4.74\times { 10 }^{ 14 }Hz\)
(iii) We find that KE of electron ejected can be increased by increasing the incident energy or by decreasing the work function. In day-to-day life we can prosper by increasing our sincere efforts or by decreasing our requirements.
