CLASS 12 – PHYSICS

Chapter 8: ADVENT OF MODERN PHYSICS

Short answer question:

Q1. What are the measurements on which two observers in relative motion will always agree upon?

Ans. The measurement on which two observers in relative motion agree is the uniform motion with respect to each other.

Q2. If the speed of light were infinite, what would the equations of special theory of relatively reduce to?

Ans. If the speed of light becomes infinity then

C = α

Then there will be no relativistic changes in time, length, and mass.

Q3. Suppose you are standing at the platform and a train passes by you. A passenger sitting in the train and you are looking at the clock on the train. Who measures the proper time interval? You or the passenger? Justify your answer.

Ans. The passenger sitting in the train measures the proper time interval because the clock is at rest with respect to him. But for the observer outside the train, the clock is in motion and having velocity of train so for the outside observer the time will dilated.

Q4. An astronaut holds a meter stick in space ship moving at high speed. What does he notice about the length of the stick as he rotates it from a position that is parallel to the line of motion to a position that is perpendicular?

Ans. As the astronaut is in the frame of reference of the spaceship in which meter stick is also lying, therefore the high speed of the spaceship will not effect the length of meter stick.

Q5. Does a light bulb at a temperature of 2500 k produce as white a light as the sun at 6000k? Explain.

Ans. According to the stefanz-boltman’s law

The intensity of the radiation α T⁴

E =σT ⁴ where σ is constant

If the light obey this law than the intensity of sun light will be more white then the intensity of bulb light. But this law is applicable to block body radiation and not light, therefore the temperature will not affect the intensity of light.

Q6. Radiation with a given wavelength causes electrons to be emitted from the surface of one metal and not from the surface of another metal. Explain why?

Ans. Each material has its own work function which is the minimum energy require to emit electron from the metal surface. This work function is given by

Ф = hc/λ

As the work function of different metal is different metal is different therefore the same radiation will not cause the emission of photoelectrons from different metal.

Q7. A beam of red light and a beam of blue light have exactly the same energy. Which light contains the greater number of photons?

Ans. From the quantum theory of light the energy of photon of photon is

E = hf

For n photons this energy will be

E = nhf

But f = c/λ

E = nhc/λ

n = Eλ/hc

As the red and blue light have same energy

So E = constant when h and c are also constant

n = constant λ

n α λ

as the wavelength of red light is greater than the wavelength of blue light, then the red light contain greater number of photons.

Q8. Beyond the threshold frequency, will light of high frequency eject greater number of photoelectrons than low frequency light?

Ans. No, the number of photoelectron does not depend upon the frequency of light, but depends upon on the intensity of light. Therefore the high frequency will not emit more photoelectrons than low frequency light.

Q10. If we keep on applying force on a material object, can it gain the speed of light?

Ans. If we keep on applying force on material object and it gain the speed of light v = c, then from relativistic expression for mass, its mass will becomes infinity and then an infinity will be required for this object to change its state which is impossible, therefore material object cannot gain the speed of light.

Q11. Can pair production take place in vacuum? Explain.

Ans. No, pair production cannot take place in vacuum. For the pair production heavy nucleus is essential to conserve the energy and momentum. As in the vacuum there is no nucleus, therefore the pair production in vacuum cannot takes place.

Q12. Why must the rest mass of a photon be zero?

Ans. If the rest mass of photon is not zero, then according to relativistic expression for mass, when photon be moving with speed of light, its mass will becomes infinity, which is not possible.

Q13. What advantages an electron microscope has over an optical microscope?

Ans. The advantage of electron microscope is that its resolving power is thousand times greater than an ordinary optical microscope. Therefore minor detail can easily be seen with the electron microscope which cannot be seen with the ordinary microscope.

Q14. Why do we say that electrons have properties? Why do we say that electrons have particle properties?

Ans. The interference and diffraction of electron beam shows that the electron has wave properties. Electron being material particle possess rest mass and hence bear the particle properties.

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