# modern physics

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```					                             D.J. Sindh Govt. Science College Karachi
Preliminary Examination B.Sc. 2005
Max: Marks: 70                                Modern Physics                               TIME: 3 HOUR

Instruction: Attempt Five questions in all selecting at least two questions from each section.
Section-A
Q1. (a) Derive Schrodinger’s time dependent wave equation. What is the significance of wave function?                 (07)
(b) Explain what do you understand by the terms Potential Well and Potential Barrier.                              (04)
(c) Explain the terms Eigen values and Eigen Function.                                                             (03)

Q2. (a) What is a Black Body? Explain Black Body Radiation. ( Explain with reference to experiment graphs)        (07)
(b) What is d’ Broglie’s wave? Explain it with the help of an experiment.                                     (05)
-6                                                       6
(c) A particle of mass 10 kg is moving along a straight line with a velocity of 10 m/s. Find its d’ Broglie’s
wave length.                                                                                              (02)

Q3. (a) What do you understand by the acronym LASER? Describe principle and construction of Helium-Neon
Laser.                                                                                          (08)
(b) How a Ruby Laser works, explain.                                                                (04)
(c) Give some applications of Lasers.                                                                (02)

Q4. (a) What is Compton Effect? Derive an expression for the Compton shift.                                    (07)
(b) There are two theories about the nature of light. Comment which Theory of Light is verified by Compton
effect.                                                                                                    (02)
(c) In an experiment of Compton effect a photon of x-ray is scattered from an electron at an angle of 90 with
the direction of incrident ray. Find the shift in wave length of the scattered photon. Given:
h = 6.63 *10-34 J.S , me = 9.1 * 10-31 kg , c = 3 * 108 m/s . All the symbols used have their usual
meaning.                                                                                                   (05)
Section-B
Q1. (a) What is radioactivity? Discuss natural radioactivity and its decay law.
(08)
(b) What do you understand by the terms “ Half Life “ and “ Mean Life “ of a radioactive subslance, explain. (02)
(c) A radioactive isotope of mercury Hg197 decays into gold Au197 with a decay constant of 0.0108 h-1.Calculate
its half life. What fraction of its original amount will remain after three half lives and after 10 days. (04)

Q6. (a) Discuss Binding Energy and Mass Dificit of a nucleus. Explain why Binding Energy per nucleon for light
and heavy nuclei are less as compared to intermediate nuclei.                                           (07)
(b) Give a brief account for the following.
(i) Nuclear Charge (ii) Nuclear Mass (iii) Nuclear Magnetic Movement (iv) Nuclear radius                 (04)
239                                                239
(c) Calculate the Binding Energy per nucleus for Pu . The needed atomic masses are 239.052158 amu (Pu ),
1.007850 amu (H1), 1.008665 amu (neutron).                                                                (03)

Q7. (a) Define the terms. (i) Fission Reaction (ii) Fusion Reaction. Discuss Fusion Reaction. What difficulties are
faced in producing Fusion Reaction in conventional Laboratories?                                               (07)
(b) Classify nuclear reactions. Give at least one example of each reaction.                                       (04)
(c) Why in case of a thermonuclear reaction D-T reactions are more favorable than D-D reactions. (03)
Q8. (a) What do you understand by Energetics of a Nuclear Reaction . Explain the importance of Q- value in a
Nuclear Reaction.                                                                                             (07)
(b) Obtain the Threshold Energy for the reaction.
Na23 ( n, ) F20
Q- value = -5.4 Mev.
(c ) Explain why extreme charge in temperature and pressure do not effect the radioactivity of a substance.         (03)

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