Chapter 30 : Einstein's theory of photo-electric effect

1. What is a photon ? Why did Einstein introduce the concept of photon ?

2. When light of wavelength 6630⁰A is incident on a metal plate whose work function is 2.2 eV will electrons be emitted or not ?

   Ans : No; as energy of incident photon

Chapter 31 : Structure of Atom

1. If an a-particle collides head-on with a stationary nucleus of Gold, then estimate the radius of the d nucleus of gold.

Chapter 32 : Bohr's Theory of Hydrogen atom and its Spectrum

1. State the three postulates of Bohr.

2. If an electron in an hydrogen atom jumps from the 4th to the 2nd permissible orbit, what will be the wave length of light emitted ? The ground state energy of hydrogen atom is -13.6 eV. If the same transition occurs in a singly ionized helium atom then what would be the wavelength ?

   Ans :

1. Compute the Compton wave length of an electron and proton.

2. Find the wave length of matter waves associated with

   (i) An electron moving at 5 106 m/s, assume that the electron at this speed is non relativistic.

   (ii) A tennis ball of mass 125 gm moving at 120 Km / hr.

   Ans : (i) 1.46 A⁰ (ii) 1.59 10^-34 m.

3. Show that angular momentum quantization of an electron in the hydrogen atom is the necessary consequence of de Broglie's matter-wave concept.

Chapter 34 : Radioactivity and Nuclear Structure

1. Half life of a radio-active substance is 1600 hours. Find its mean life and decay constants. After how many hours will 75% of the substance decay radioactively ?

2. Give examples of physical processes obeying the exponential law.

Chapter 35 : Einstein's Theory of Special Relativity

1. A rod lying on x-axis of a frame S and at rest relative to it has length 1m. What would be its length measured by an observer at rest in S' frame which is moving along the x-axis of S frame with uniform speed of (i) 10⁴ m/s (ii) 10⁶ m/s (iii) 10⁸ m/s.

   Ans : 0.999997 m, 0.999994 m, 0.94281 m

2. If the helium nucleus is completely dismembered then how much energy would be released ?

   Ans : 0.504 10^-11 Joule or 32.156 MeV

3. "If one could travel at the speed of light, then the person would become immortal" explain whether statement is true or false.

   Ans : True, because i.e. no time elapses if one could travel at the speed of light.

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