Question: 1

Give two postulate of special theory of relativity.

Answer: 1

1-38

Postulate of special theory of relativity are states as

The laws of physics are the same in all inertial frames.
The speed of light in free space has the same value for all observes,regardless of their state of motion.

Question: 2

Why can red light be used in a photographic dark room when developing films,but a blue or white light can not?

Answer: 2

2-38

As we known red light has longest wavelength in visible spectrum,therefore it has less energy than that of blue or white light.So red light is least scattered on account of its large wavelength.Hence photographic films and the materials concerned are less affected in the presence of red light than high energy blue of white light.

Question: 3

Why don't we observe a Compton Effect with visible light?

Answer: 3

3-38

The frequency of visible light is less than x rays,And the wavelength of visible light is much greater than the Compton of wavelength of electron,So compton effect can not observed with visible light.

Question: 4

Does the dilation mean that time really passes more slowly in moving system or that it only seems to pass more slowly.Explain briefly.

Answer: 4

4-38

According to dilation formula

t=

\scriptstyle {\sqrt {1-{\frac {v^{2}}{c^{2}}}}}

"

This relation show that a clock moving with respect to an observe appears to move slow then it does when it is at rest with respecttohim.So the moving clocks just appears to run slowly to the observer at rest.Hence time dilation is an apparent change and it only seems to pass more slowly but not actually.

Question: 5

A satellite is orbiting around earth.Is its frame of reference inertial or non-inertial ?Justify your answer.

Answer: 5

5-38

The motion of the satellite is synchronized with the earth so it is in the same frame of reference in which earth lies.Hence the frame of reference will be inertial.

Question: 6

The rest mass of photon is zero.Is its momentum also zero?

Answer: 6

6-38

No,The rest mass of photon is zero.It travels in the form of small energy packets which are of equal wavelength .When it is in motion,it possesses some mass.So it which it will also have momentum.

Question: 7

Differentiate between inertial and non-inertial frame of reference.

Answer: 7

7-38

A coordinate system in which the law of inertia is valid is called inertial frame of reference.It is non-accelerated frame of reference.

Question: 8

As a solid is heated and begins to glow ,why does it first of its absolute.

Answer: 8

8-38

Since the red light has longest wavelength ,so it will be emitted first and solid appears red first.

Question: 9

What are advantages of NAVSTAR navigation system?

Answer: 9

9-38

NAVSTAR is used to locate the position and find the speed of an object anywhere on Earth up to to an accuracy of 2 cms^-1.

Question: 10

Can an object move with the speed of light?

Answer: 10

10-38

As an object approaches the speed of light,its masses rises abruptly.If an object tries to travel 186,000 miles per seconds,its mass become infinite and so does the energy required to move it, For this reasons ,no normal object can be travel as fast as or faster than the speed of light.

Question: 11

What are the measurement on which two observers in relative motion will always agree upon?

Answer: 11

11-38

Two observe in relative motion will always agree upon

Force
Acceleration

Question: 12

If you are moving in a spaceship at very high speed relative to the earth.Would you notice difference(a)In your pulse rate(b)In the pulse rate of people on the earth?

Answer: 12

12-38

The pulse rate of the person inside the spaceship moving with large velociety will decrease.
The pulse rate of the people on the earth with respect to the person inside the spaceship with large velocity will increase.

Question: 13

Define Stopping potential and Threshold frequency.

Answer: 13

13-38

Stopping potential:The stopping potential is the potential difference applied to stop the electrons from being ejected from the surface when the light fall on it.

Threshold frequency:The minimum of frequency of incident light at which electrons are emitted from a surface is called threshold frequency.

Question: 14

Distinguished between photoelectric effect and Compton effect.

Answer: 14

14-38

The emission of electron from a metal surface when exposed to suitable frequency light is called photoelectric effect and when x rays are scattered by loosely bound electrons from graphite target the phenomena of change in wavelength is known as compton effect.

Question: 15

Define Compton effect.

Answer: 15

15-38

The phenomena of increase in wavelength of x rays photon scattered by loosely bound electrons from a graphite target is called Compton Effect.

Question: 16

What are conclusion made from the pair production?

Answer: 16

16-38

Pair production is the creation of an elementary particle and its antiparticle,Pair production offers refers specifically to a photon creating an electron positron pair near a nucleus but can more generally refer to any particle antiparticle pair creation.

Energy can be converted into mass according to E=mc²

Question: 17

Define ionization potential and excitation potential.

Answer: 17

17-38

Ionization potential:The potential necessary to remove an electron from the atom is called ionization potential,It is expressed in volts.

Question: 18

Will higher frequency of light ejects greater number of electrons than low frequency of light?

Answer: 18

18-38

No it will not,The number of ejected electrons depend upon the intensity of light.They are independently of frequency.

Question: 19

Will the bright light ejects more from metal surface than dimmer light of tha same colour?

Answer: 19

19-38

Since intensity

number of electrons.

and bright lightis more intensethan dimmer one.So bright light will ejects more electrons than dimmer light.

Question: 20

Define Compton effect.Write formula of Compton shift for scattering angle.

Answer: 20

20-38

When X-rays are scattered by loosely bound electrons from a graphite target,the phenomena of change in wavelength is known as Compton Effect.Compton shift for scattering angle is given

\lambda '-\lambda ={\frac {h}{m_{e}c}}(1-\cos {\theta }),

Question: 21

Define pair production and annihilation of matter.

Answer: 21

21-38

Pair production:The change of very high energy photon into an electron,positron pair is called pair production.

Annihilation of matter:When a positron comes close to an electron,they annihilation and produce two photons in the gamma rays range.It is called annihilation of matter.

Question: 22

What do you understand by work function and stopping potential?

Answer: 22

22-38

The minimum amount energy required to remove an electron from the surface of a metal is called work function.

And the stopping potential is the potential difference applied to stop the electrons from being ejected from the surface when the light falls on it.

Question: 23

Photoelectric effect give the evidence of the particle nature of light,Explain it how?

Answer: 23

23-38

If light were simple a wave like phenomena ,then increasing the intensity and thereby increasing the total energy failing on the surface would be expected to eventually provide enough energy to release electron no matter what the frequency .Furthermore in the classical picture one would expect the maximum energy of the emitted electrons to depend on the intensity of light--but it does not.So this is evidence that light behaves as if it were a particle.

Question: 24

Is it possible to create a single electron from energy?

Answer: 24

24-38

No it is not possible to create single electron from energy.Because electron has negative charge and photon has no charge,

Question: 25

When does light behaves as a wave?When does light behaves as a particle?

Answer: 25

25-38

Light behave as a wave when it propagates from one place to other and light behave as a particle when it interacts with matter,Light behaves as a wave in interference and diffraction,Light behaves as a particle in photoelectric and Compton effect.

Question: 26

Write at least two justification for light to behave as wave and as a particle.

Answer: 26

26-38

Interference and diffraction confirms wave nature of light while photoelectric effect and compton effect confirms particle nature of light.

Question: 27

What are black body radiations and how can you get a black body?

Answer: 27

27-38

An objects that absorbs all radiations falling on it,at all wavelength is called a black body.

When a body is heated ,it emits radiation its emission is called black body.

Black body is solid that has hollow cavity within it and a small hole through which radiations can enter or escape.The inside is blackened with suit to make it as good an absorber and as a bad reflector as possible.

Question: 28

We do not notice the de-Broglie wavelength for a pitched cricket ball.Explain why?

Answer: 28

28-38

According to de-Broglie hypothesis

λ =h/mv

As cricket ball has large mass,therefore wavelength of wavelength associated with it is so small that is not detectable.

Question: 29

What do you know by annihilation of matter?

Answer: 29

29-38

When a positron comes close to an electron,they annihilate and produce two photons in the gamma rays range.It is called annihilation of matter.

Question: 30

When light shine on surface,is momentum transferred to metal surface?

Answer: 30

30-38

Yes when light shine on surface,momentum is transferred to metal surface and also the energy.So metal is heated up.

Question: 31

State uncertainty principle.

Answer: 31

31-38

The product of uncertainty in the measurement of momentum and uncertainty in the measurement of position of an electron is approximately equal to Planck's constant.

The product of uncertainty in the measurement of energy and uncertainty in the measurement of time of an electron is approximately equal to Planck's constant.

Question: 32

Which has the lower energy quanta,radio waves or x rays?Explain

Answer: 32

32-38

Energy of quanta is given as E=hf-hc/lamda

Radio waves length has longer wavelength,Therefore radio waves has lower energy quanta.

Question: 33

Differentiate between special theory of relativity and general theory of relativity.

Answer: 33

33-38

The special theory of relativity treats problem involving inertial or non accelerating frame of reference.

Question: 34

What is wave particle duality?

Answer: 34

34-38

It says that light has dual nature;it travels in the form of waves but interacts with matter in the form of energy particle called as photon.e.g in interference ,reflection and polarization it show the properties of wave nature but in phenomena like pair production and compton's effect light acts as energy particle.

Question: 35

Can a pair production take place in vacuum?Explain

Answer: 35

35-38

No,pair production can not take place in vacuum.

In order to conserve the momentum and energy,their presence of heavy nucleus is essential,The vacuum has no particle or heavy nucleus,Therefore pair production can not take place in vacuum.

Question: 36

What advantage of an electron microscope has over an optical microscope?

Answer: 36

36-38

Resolving power of an electron microscope is 1000 times greater than that of optical microscope.
Magnification of an electron microscope is also about 1000 times greater than that of optical microscope.
3-D image of remarkable quality can be obtained by electron.

Question: 37

Is it possible to create a single electron from energy?Explain

Answer: 37

37-38

No it is not possible to create a single electron from energy.Creation of single electron will be against the law of conservation of charge and the law of conservation of momenteum.In pair production an electron positron pair is produces.

Question: 38

Define work function and threshold frequency.

Answer: 38

38-38

The minimum amount of energy required to remove electrons from a metal surface is called work function of this metal.

The minimum frequency below which photoelectric effect can not occur from a metal surface is called threshold frequency of this metal.

Physics Chapters

12th Class Chapter 8 Physics Short Questions

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