Answers to Review Questions: Ch # 17 Dawn Of Modern Physics

XII BSEK ( Karachi Board)

Note:- For 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, 17.7 ,17.12 see theory notes

Q.17.8 Why Compton Effect not observable with visible light?
Ans. In Compton effect ,x rays of wave length =1 Ao equivalent to energy=140 ev , were directed on the graphite block, where binding energies of bounded electrons are =100 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.17.9 What phenomenon……………..related quantitatively?
Evidences for wave Phenomenon
Evidences for matter phenomenon
Interference of light
Black body radiation
Diffraction of light
Photo electric effect
Polarization of light
Compton effect
Optical Doppler’s effect
x ray production
Prod. Of electromagnetic waves


Q.17.10 In what way the …….  and protons?
Characteristics of particles of matter
Characteristics of light (photons)
1.Non zero rest mass
1.Zero rest mass
2. They possess inertia
2.They consists of energy packets
3. Speed is always less than speed of light
3. They travel with the speed of light
4. They may be charged or uncharged
4. They are always neutral
5.They are guided by matter waves
5. No relation with the matter waves
6.E=1/2 mv2
6. E= hν

Q.17.11 In the photo electric effect the energy……..Explain.
Ans. When radiation strikes a metal surface, it deposits its entire energy on some electron in the absorbing surface>if energy of the photon (E= hν) exceeds the energy expend by the electron in work against the force binding it to the surface (φ) it will emitted with some energy. K.e < hν

Q.17.13 with the help of ………. Of an atom.
Ans. A/c to de broglie hypothesis, an electron moving around nucleus is pictured as a kind of wave packet. An electron can circle a nucleus indefinitely without radiating energy provided that its orbit contains an integral number of de broglie wavelength.


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