ADVENT OF MODERN PHYSICS
Chapter - 17
Q.1: What do you understand by a frame of reference? What is the difference between inertial frame and non-inertial frame?
Ans: The position and motion of a body can be located with reference to some coordinate system, called the frame of reference.
The frame of reference that is either at rest or moves with uniform velocity is called an inertial frame. It has zero linear or rotational acceleration. Newton’s laws hold well in such a frame. All inertial frames of references are equivalent.
The frame of reference which possesses acceleration is known as a non-inertial frame. Laws of motion do not remain valid in such a frame.
Q.2: Explain why the Compton Effect is not observable with visible light?
Ans: In Compton’s experiment of x-rays of wavelength - 1 Å, equivalent to energy - 140 eV, were directed on a graphite block, where binding energies of bounded electrons were 102 eV. If visible light is used, it possesses low frequency, and these photons have energies - 0.1 eV. This energy is too small to be given to loosely bound electrons to get them scattered.
Q.3: What phenomena require wave description of light? What phenomena required particle picture of light? How are the two aspects related putatively?
Ans: The convincing evidences that light are a wave phenomena are:
- Interference of light
- Diffraction of light
- Polarization of light
- Production of electromagnetic waves
- The optical Doppler Effect.
The idea of quantum nature of light, i.e., photon (which has a particle nature), was introduced due to the following evidences:
- Black body radiation
- The photoelectric emission
- Compton scattering
- X-ray production
The wave and particle aspects are related in de Broglie equation as: . Thus a particle of non-zero rest mass moves as if it were guided by an associated matter wave. Nevertheless, the ‘particle waves’ are waves of probability. Confirmation of de Broglie wavelength came in 1927 by C.J. Division and L.H. German and, independently, G.P. Thomson. It is astounding to note that Thomson (J.J.), the father, was awarded the Nobel prize in 1906 for having shown that the electron is a particle; and 31 years later Thomson (J.P.), the son, for having shown that the electron is a wave.
Q.4: In what way do the particles of light (photons) differ from the particles of matter, such as electrons and protons?
Ans:
Particles of matter (e.g., electron, proton, etc.) possess certain characteristics:
- Non-zero rest mass.
- They possess inertia and contain no energy 'packets.'
- Their speed is always less than (speed of light).
- Their energy is proportional to the square of the speed ().
- They may be charged or uncharged.
- When in motion, they are guided by matter waves.
Particles of light (photons) possess the following distinct characteristics:
- Zero rest mass.
- They consist of waves in packets of discrete amounts (called 'energy packets' or quanta).
- They travel with speed equal to that of light.
- Their energy is proportional to frequency ().
- They are always electrically neutral.
- They are not guided by matter waves.
Q.5: In the photoelectric effect, the energy of a photoelectron is less than that of an incident photon. Explain?
Ans: When radiation (a photon) strikes a metal surface, it deposits its entire energy on some electron in the absorbing surface. If the energy of the photon (by ) exceeds the energy required by the electron in work against the force binding it to the surface (), it will be emitted with some energy. As , hence .
Q.6: How did de Broglie hypothesis help to explain the stability of the atom?
Ans: According to de Broglie's hypothesis, an electron moving around the nucleus is pictured as a kind of wave packet (standing wave). An electron can circle a nucleus indefinitely without radiating energy provided that its orbit contains an integral number of de Broglie wavelengths.
Q.7: What is rest mass of a body?
Ans: Rest mass () is the mass of a body when it is at rest with respect to an observer. The relativistic mass of a body moving with certain velocity is given by
Q.8: If we keep applying a force on a material object, can the object gain the speed of light?
Ans: If we keep applying a force which can produce a velocity equal to the velocity of light (), then the mass of the material body would become infinite. This is not possible.
Q.9: A block of polished metal having a black spot in the middle is heated above 3000 K and then placed in a dark room. Write your observations?
Ans: If a metal block is heated to incandescence at about 1000K, the metal has a dull red glow. A further high temperature changes into orange, then yellow and finally to white (3000 K) the black spot behaves as a black body. It absorbs maximum energy and appears as black. When seen in a dark room, the black spot radiates more energy (since a good emitter is a good emitter) than the rest of the block. The black spot appears brighter than the rest of the surface.
Q.10: Does the fact that an atom can emit a photon violate the law of conservation of energy? Explain?
Ans: No. An atom in an excited state can emit a photon. The energy received in jumping up is released in the emission.
Q.11: Can matter (e.g., electron) be created or destroyed?
Ans: Matter can be created from energy (photon) in pair production; and can be destroyed as photons in annihilation of matter process.
Q.12: Can pair production take place in vacuum? Explain
Ans: No, because this process requires a heavy nucleus to conserve momentum and energy of the system. The heavy nucleus takes the recoil after stopping the photon.
Q.13: Can an intense beam of television waves focused on a metal cause photoemission?
Ans: No, because TV waves are of low frequency, while a metal requires high threshold frequency for photoemission.
Q.14: Both photoelectric emission and Compton scattering are processes that involve interaction of radiation with matter. How do they differ?
Ans: In photoelectric effect, a low energy photon (e.g., ultraviolet light) can lose all its energy on striking an electron, and the photon vanishes. But in Compton Effect, a high energy photon (e.g., X-ray) loses part of energy and a photon is scattered with the remaining energy (and hence frequency decreases).
Q.15: The speed limit of our highways is 65 km/h. If the speed of light were the same, would you be able to drive at the speed limit?
Ans: No, because mass would become infinite.
Q.16: A ball is dropped from a helicopter flying at constant speed horizontally. Describe its motion relative to the pilot and an observer on earth’s surface.
Ans: According to the observer on the earth, it will fall forward towards the ground following a projectile path.
According to a pilot, it will to a point on the earth vertically.
Q.17: Why does the casing of a large electric transformer have metal blades fastened to it perpendicular to the surface and painted black?
Ans: The blades transfer heat by radiation to the atmosphere by increasing the surface area. They are painted black because black body radiates energy at a faster rate.
