JEE Main Previous Year Papers Questions With Solutions Physics Modern Physics

JEE Main Previous Year Papers Questions With Solutions Physics Modern Physics

Multiple Choice with ONE correct answer
1.An alpha particle of energy 5 MeV is scattered through 180° by a fixed uranium nucleus. The distance of closest approach is of the order of [1981-2Marks]

Ans.

2.The half life of radioactive radon is 3.8 day. The time at the end of which (l/20)^th of the radon sample will remain undecayed is (given log₁₀e= 0.4343) [1981-2Marks]
a)8 day   b) 16.5 day     c) 33 day    d) 76 day
Ans.

3.Beta rays emitted by a radioactive material are  [1983-1Mark]
a)electromagnetic radiations
b)the electrons orbiting around the nucleus
c)charged particles emitted by the nucleus
d)neutral particles.
Ans.

4.If elements with principal quantum number 4 were not allowed in nature, the number of possible elements would be         [1983-lMark]
a)60        b) 32           c) 4           d) 64
Ans.

5.Consider the spectral line resulting from the transition n = 2 -> n = 1 in the atoms and ions given below. The shortest wavelength is produced by [1983-l Mark]
a)hydrogen atom  b) deuterium atom  c) singly ionized helium  d) doubly ionised lithium
Ans.

Ans.

7.Fast neutrons can easily be slowed down by :
a)the use of lead shielding
b)passing them through water
c)elastic collisions with heavy nuclei
d)applying a strong electric field
Ans.(b) Fast neutrons can easily be slowed down by passing them through water. Heavy water is used as moderator in nuclear reactors.

8.Consider a – particles, b – particles & g rays, each having an energy of 0.5 MeV. Increasing order of penetrating power of the radiations are : [JEE’94]
a)a, b, g b) a, g, b c) b, g, a d) g, b, a
Ans.

9.An energy of 24.6 eV is required to remove one of the electrons from a neutral helium atom. The energy (In eV) required to remove both the electrons form a neutral helium atom is: [JEE’95]
a)2    b) 49.2    c) 51.8    d) 79.0
Ans.

10.A radioactive material decays by simultaneous emission of two particles with respective half-lives 1620 and 810 year. The time, in year, after which one-fourth of the material remains is [1995]
a)1080    b) 2430    c) 3240   d) 4860
Ans.

Ans.

Ans.

13.As per Bohr model, the minimum energy (in eV) required to remove an electron from the ground state of doubly ionized Li atom (Z = 3) is
a)51 b) 13.6        c) 40.8        d) 122.4
Ans.

14.The maximum kinetic energy of photoelectrons emitted from a surface when photons of energy 6 eV fall on it is 4 eV. The stopping potential in Volts is :
a)2          b) 4            c) 6            d) 10
Ans.(b) Stopping potential is the negative potential which stops the emission of (K.E.)_max electrons when applied.

Stopping potential = 4 volt.

15.In Hydrogen spectrum the wavelength of H_a line is 656 nm, where as in the spectrum of a distant galaxy, H_a line wavelength is 706 nm. Estimated speed of the galaxy with respect to earth is,[1999-2Marks]
a)2xl0⁸m/s     b) 2xl0⁷m/s
c) 2xl0⁶m/s    d) 2xl0⁵m/s
Ans.

16.A particle of mass M at rest decays into two particles of masses m1 and m₂, having non-zero velocities. The ratio of the de-Broglie wavelengths of the particles, l_1/ l₂, is  [JEE ’99]
a)m1/ m₂ b) m₂/m₁ c) 1.0 d) Om₂/Om1
Ans.

17.Which of the following is a correct statement?
a)Beta rays are same as cathode rays
b)Gamma rays are high energy neutrons.
c)Alpha particles are singly ionized helium atoms
d)Protons and neutrons have exactly the same mass
Ans.(a) a) Beta rays are same as cathode rays as they are streams of electrons. The statement is correct.(b)Gamma rays are electromagnetic waves They are not neutrons

(c)Alpha particles are doubly ionised helium atoms, not singly ionised heliumatoms.

(d)Protons have mass slightly less than that of neutrons. Masses are not exactly the same. Option (b), (c) and (d) are incorrect.Option (a) is correct.

Ans.(b) Nuclear density is of the order of 10¹⁷ kg/m³

^19.22Ne nucleus, after absorbing energy, decays into two a-particles and an unknown nucleus. The unknown nucleus is
a)nitrogen b) carbon c) boron d) oxygen
Ans.

Ans.

Ans.

22.The electron in a hydrogen atom makes a transition from an excited state to the ground state. Which of the following statements is true? [JEE’ 2000 (Scr)]
a)Its kinetic energy increases and its potential and total energies decrease.
b)Its kinetic energy decreases, potential energy increases and its total energy remains the same.
c)Its kinetic and total energies decrease and its potential energy increases.
d)Its kinetic, potential and total energies decrease.
Ans.

23.Two radioactive materials X₁ and X₂ have decay constants 101 and 1 respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of X₁ to that of X₂ will be 1/ e after a time of        [JEE ’ 2000 (Scr)]
a)1/(101)         b) 1/(111)
c) 11/(101)      d) 1/(91)
Ans.

24.Electrons with energy 80 ke V are incident on the tungsten target of an X-ray tube. K shell electrons of tungsten have 72.5 keV energy. X-rays emitted by the tube contain only                                                    [2000-2Marks]
a)a continuous X-ray spectrum (Bremsstrahlug) with a minimum wavelength of 0.155 A⁰
b)a continuous X-ray spectrum (Bremsstrahlung) with all wavelengths
c)the characteristic X-ray spectrum of tungsten
d)a continuous X-ray spectrum (Bremsstrahlung) with a minimum wavelength of 0.155 A⁰ and the characteristic X-ray spectrum of tungsten.
Ans.

25.The electron emitted in beta radiation originates from[JEE’2001(Scr)]
a)inner orbits of atoms
b)free electrons existing in nuclei
c)decay of a neutron in a nucleus
d)photon escaping from the nucleus
Ans.

26.The transition from the state n = 4 to n = 3 in a hydrogen like atom results in ultraviolet radiation. Infrared radiation will be obtained in the transition [2001-2Marks]
a) 2—>l  b) 3 —>2     c) 4 —>2     d)5—>4
Ans.

Ans.

Ans.

29.The potential difference applied to an X – ray tube is 5 kV and the current through it is 3.2 mA. Then the number of electrons striking the target per second is         [JEE’ 2002 (Scr.)]
a)2 x 10¹⁶ b) 5 x 10¹⁶ c) 1 x 10¹⁷ d) 4 x 10¹⁵
Ans.

Ans.

Ans.

Ans.

Ans.

Ans.

Ans.

Ans.

37.In a photoelectric experiment anode potential is plotted against plate current           [2004-2 Marks]
a)A and B will have same intensities while B and C will have different frequencies
b)B and C will have different intensities while A and B will have different frequencies
c)A and B will have different instensities while B and C will have equal frequeancies
d)B and C will have equal intensities while A and B will have same frequencies.
Ans.

38.A 280 day old radioactive substance shows an activity of 6000 dps, 140 day later its activity becomes 3000 dps. What was its initial [2000-2marks]
a) 20000 dps b) 24000 dps
c) 12000 dps d) 6000 dps
Ans.

Ans.

Ans.

41.A photon of 10.2 eV energy collides with a hydrogen atom in ground state inelastically. After few microseconds one more photon of energy 15 eV collides with the same hydrogen atom.Then what can be detected by a suitable detector. [JEE’2005 (Scr)]
a)one photon of 10.2 eV and an electron of energy 1.4 eV
b)2 photons of energy 10.2 eV
c)2 photons of energy 3.4 eV
d)1 photon of 3.4 eV and one electron of 1.4 eV
Ans.

42.A beam of electrons is used in an Young double slit experiment. The slit width is d. When the velocity of electron is increased, then [2005]
a)no interference is observed
b)fringe width increases
c)fringe width decreases
d)fringe width remains same
Ans.

43.If a star can convert all the He nuclei completely into oxygen nuclei, the energy released per oxygen nuclei is [Mass of He nucleus is 4.0026 amu and mass of oxygen nucleus is 15.994 amu]
a) 7.6 MeV        b) 56.12 MeV
c) 10.24 MeV     d) 23.9 MeV
Ans.

44.Given a sample of Radium-226 having half-life of 4 days. Find the probability, a nucleus disintegrates within 2 half lives.            [JEE 2006]
a) 1     b) 1/2     c) 3/4    d) 1/4
Ans.

45.The largest wavelength in the ultraviolet region of the hydrogen spectrum is 122 nm. The smallest wavelength in the infrared region of the hydrogen spectrum (to the nearest integer) is [2007-3Marks]
a) 802 nm            b) 823 nm
c) 1882 nm          d) 1648 nm
Ans.

46.In the options given below, let E denote the rest mass energy of a nucleus and n a neutron. The correct option is  [2007-3Marks]

Ans.(a) Rest mass energy of U will be greater than the rest mass energy of the nuclei into which it breaks. The constituent nuclei and neutrons will have kinetic energy also, as a result of conservation of linear momentum.

Ans.

48.Which one of the following statements is WRONG in the context of X-rays generated from a X-ray tube? [2008]
a)Wavelength of characteristic X-rays decreases when the atomic number of the target increases
b)Cut-off wavelength of the continuous X-rays depends on the atomic number of the target.
c)Intensity of the characteristic X-rays depends on the electrical power given to the X-ray tube
d)Cut-off wavelength of the continuous X-rays depends on the energy of the electrons in the electrons in the X-ray tube.
Ans.

Ans.

Ans.

Ans.

Multiple Choice with ONE or  More Than ONE correct answers
52.The shortest wavelength of X-rays emitted from an X-ray tube depends on [1982-3Marks]
a)the current in the tube
b)the voltagae applied to the tube
c)the nature of the gas in tube
d)the atomic number of the target material
Ans.

53.The threshold wavelength, for photoelectric emission, from a material is 5200 A⁰. Photo­electrons will be emitted when this material is illuminated with monochromatic radiation from a[1982-3Marks]
a)50 watt infrared lamp
b)1 watt infrared lamp
c)50 watt ultraviolet lamp
d)1 watt ultraviolet lamp.
Ans.

Ans.(b),(c) Nuclear fusion reaction occurs when two or more lighter nuclei combine to produce a heavier nucleus.

Ans.

56.The X-ray beam coming from an X-ray tube will be [1985-2Marks]
a)monochromatic
b)having all wavelengths smaller than a certain maximum wavelength
c)having all wavelength larger than a certain minimum wavelength
d)having all wavelengths lying between a minimum and a maximum wavelength
Ans.

57.The mass number of a nucleus is [1986-2Marks]
a)always less than its atomic number
b)always more than its atomic number
c)sometimes equal to its atomic number
d)sometimes more than and sometimes equal to its atomic number.
Ans.

58.Photoelectric effect supports quantum nature of light because     [1987-2Marks]
a)there is a minimum frequency of light below which no photoelectrons are emitted
b)the maximum kinetic energy of photo electrons depends only on the frequency of light and not on its intensity.
c)even when the metal surface is faintly illuminated, the photoelectrons leave the surface immediately
d)electric charge of the photoelectrons is quantized.
Ans.(a),(b),(c)

59.During a negative beta decay [1987-2Marks]
a)an atomic electron is ejected
b)an electron which is already present within the nucleus is ejected
c)a neutron in the nucleus decays emitting an electrons,
d)a part of the binding energy of the nucleus is converted into an electron.
Ans.

60.During a nuclear fusion reaction [1987-2 Marks]
a)a heavy nucleus breaks into two fragments by itself.
b)a light nucleus bombarded by thermal neutrons breaks up
c)a heavy nucleus bombarded by thermal neutrons breaks up
d)two light nuclei combine to give a heavier nucleus and possibly other products.
Ans.(d)

61.The potential difference applied to an X-ray tube is increased. As a result, in the emitted radiation
a)the intensity increases [1988-2Marks]
b)the minimum wavelength increases
c)the intensity remains unchanged
d)the minimum wavelength decreases
Ans.

62.A freshly prepared radioactive source of half life 2 hr emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safely with this source is     [1988-2Marks]
a) 6 hr   b) 12 hr    c) 24 hr   d) 28 hr
Ans.

Ans.

64.When a monochromatic point source of light is at a distance of 0.2 m from a photoelectric cell, the cut off voltage and the saturation current are respectively 0.6 V and 18.0 mA. If the same source is placed 0.6m away from the photoelectric cell, then   [1992-2Marks]
a)the stopping potential will be 0.2 volt
b)the stopping potential will be 0.6 volt
c)the saturation current will be 6.0 mA
d)the saturation current will be 2.0 mA.
Ans.(b),(d) The stopping potential depends on the frequency of incident radiation and not on the intensity of incident radiation. The source is same, though placed at different distances, consequently the stopping potential remains unchanged. The saturation current depends on the radiation- intensity, incident on cathode of photocell. The radiation-intensity depends on the distance between the source and the cathode. Infact, inverse square law applies to intensity.

Ans.

Ans.

 

67.Which of the following statement(s) is (are) correct ?  [JEE’94]
a)The rest mass of a stable nucleus is less than the sum of the rest masses of its separated nucleons.
b)The rest mass of a stable nucleus is greater than the sum of the rest masses of its separated nucleons.
c)In nuclear fusion, energy is released by fusion two nuclei of medium mass (approximately 100 amu).
d)In nuclear fission, energy is released by fragmentation of a very heavy nucleus.
Ans.(a),(d)

Ans.(c),(d) M, is the mass of ₁₀Ne²⁰
M₂ is the mass of ₂₀Ca⁴⁰ nucleus.
Ne means 10 protons + 10 neutrons.
Ca means 20 protons + 20 neutrons.
Protons and neutrons are constituent particles. Due to mass defect, which is necessary for bind­ing the nucleus, mass of nucleus is always less than the sum of masses of protons and neutrons.

Ans.

Ans.

Ans.

72.
Ans.

73.The half-life period of a radioactive element X is same as the mean-life time of another radioactive element Y. Initially both of them have the same number of atoms. Then     [JEE ’99]
a)X & Y have the same decay rate initially
b)X & Y decay at the same rate always
c)Y will decay at a faster rate than X
d)X will decay at a faster rate than Y

75.Assume that the nuclear binding energy per nucleon (B/A) versus mass number (A) is as shown in the figure. Use this plot to choose the correct choice (s) given below.

76.Statement-1 : If the accelerating potential in an X-ray tube is increased, the wavelengths of the characteristic X-ray do not change.
Statement-2 : When an electron beam strikes the target in an X-ray tube, part of the kinetic energy is converted into X-ray energy.
a)Statement-1 and 2 are true and statement-2 is a correct explanation for statement-1
b)Statement-1 and 2 are true and statement-2 is not a correct explanation for statement-1
c)Statement-1 is true, statement-2 is false
d)Statement-1 is false, statement-2 is true.
Ans.(b)

Comprehension based question 

Read the passage given below and answer the questions that follow

Passage – 1 : In a mixture of H-He⁺ gas (He⁺ is sinagly ionized He atom), H atoms and He⁺ ions are excited to their respective first excited states. Subsequently, H atoms transfer their total excitation energy to He⁺ions (by collisions). Assume that the Bohr model of atom is exactly valid.   [2007-3Marks]

77.The quntum number n of the state finally populated in He⁺ ions is

a) 2    b) 3 c) 4             d) 5

Ans.

Ans.

79.The ratio of the kinetic energy of the n = 2 electrons for the H atom to that of He⁺ ion is[2008]
a) 1/4     b) 1/2        c) 1     d) 2
Ans.

Passage – 2 : The key feature of Bohor’s theory of spectrum of hydrogen atom is the quantization of an­gular momentum when an electron is revolving around a proton. We will extend this to a general rotational motion to find quantized rotational energy of a diatomic molecule assuming it to be rigid. The rule to be applied is Bohr’s quantization condition.

80.A diatomic molecule has moment of inertia I. By Bohr’s quantization condition its rotational energy in the n^th level (n = 0 is not allowed) is

Ans.

Ans.

Ans.

Passage – 3 : When a particle is restricted to move along x-axis between x = 0 and x = a, where a is of nanometer dimension, its energy can take only certain specific values. The allowed energies of the particle moving in such a restricted region, correspond to the formation of standing waves with nodes at its ends x = 0 and x = a. The wavelength of this standing wave is related to the linear momentum p of the particle according to the de Broglie relation. The energy of the particle of mass m is related to its linear momentum as

83.The allowed energy for the particle for a particular value of n is proportional to
a) ar²        b) a^-3/2          c) a^-1     d) a²
Ans.

84.If the mass of the particle is m = 1.0 x 10-³⁰kg and a = 6.6nm, the energy of the particle in its ground state is closest to
a) 0.8 meV        b) 8 meV     c) 80 meV       d) 800 meV
Ans.

85.The speed of the particle, that can take discrete values, is proportional to
a) n-^3/2    b) n^-1    c) n^1/2           d) n
Ans.

Ans.
(d) Plasma state is achieved at high temperature

Ans.

Ans.

Ans.

Ans.

Matching Type

Ans.a—>p, b —>s, c—>r, d—>q

Ans.

Ans.

Ans.

Ans.

Subjective / Numerical integer type    
96.How many electrons, protons and neutrons are there in a nucleus of atomic number 11 and mass number 24? [1982-2Marks]
Ans.

97.A uranium nucleus (atomic number 92, mass number 238) emits an alpha particle and the resultant nucleus emits (3 -particle. What are the atomic number and mass number of the final nucleus?         [1982-2Marks]
Ans.

Ans.

Ans.
 

Ans.

101.A double ionised lithium atom is hydrogen-like with atomic number 3.    [1985-6Marks]
i)Find the wavelength of the radiation required to excite the electron in Li from the first to the third Bohr orbit.(Ionisation energy of the hydrogen atom equals 13.6eV).
ii)How many spectral lines are observed in the emission spectrum of the above excited system?
Ans.

102.There is a stream of neutrons with a kinetic energy of 0.0327 eV. If the half life of neutrons is 700 second, what fraction of neutrons will decay before they travel distance of 10 m? [1986-6 Marks]
Ans.
 

Ans.

Ans.

105.A gas of identical hydrogen – like atoms has some atoms in the lowest (ground) energy level A and some atoms in a particular upper (excited) energy level B and there are no atoms in any other energy level.The atoms of the gas make transition to a higher energy level by absorbing monochromatic light of photon energy 2.7eV. Subsequently, the atoms emit radiation of only six different photon – energies. Some of the emitted photons have energy 2.7eV, some have energy more and some have less than 2.7eV.
i)Find the principal quantum number of the initially excited level B.
ii)Find the ionization energy for the gas atoms.
iii)Find the maximum and the minimum energies of the emitted photons.   [1989-8 Marks]
Ans.

106.Electrons in hydrogen like atoms (Z=3) make transitions from the fifth to the fourh orbit and from the fourth to the third orbit. The resulting radiations are incident normally on a metal plate and eject photo electrons. The stopping potential for the photo electrons ejected by the shorter wavelength, is 3.95 volt. Calculate the work function of the metal and the stopping potential for the photo electrons ejected by the longer wavelength. (Rydberg constant = 1.094 x 10⁷m-1) [1990-71 marks]
Ans.

Ans.

108.A mono chropiatic point source radiating wave length 6000 A, with powev 2 watt, an aperture A of diameter 0.1 m and a lavge screen SC are placed as shown in fig. A photo emissive detector D of surface area 0.5 cm² is ‘placed at the centre of the screen. The efficiency of the detector for the photo electron generation per incident photon is 0.9.
a)Calculate the photon flux at the centre of the screen and the photo current in the detector.
b)If the concave lens L of focal length 0.6 m is inserted in the aperture as shown, find the new values of photon flux and photo current. Assume a uniform average transmission of 80% from the lens.
c)If the work function of the photo emissive  sur­face is leV, calculate the values of the stopping potential in the two cases (without and with the lens in the aperture). [1991-2+4+2 Marks]

Ans.

109.Photo electrons are emitted when 400 nm radiation is incident on a surface of work function 1.9eV. These photo electrons passes through a region containing a-particles. A maximum energy electron combines with an a-particle to form a He⁺ ion, emitting a single photon in this process. He⁺ ions thus formed are in their fourth excited state. Find the energies in eV of the photons, lying in the 2 to 4eV range, that are likely to be emitted during and after the combination. [Take, h = 4.14 ’10^-15 eV-s ][JEE ’99]
Ans.

110.A hydrogen – like atom of atomic number Z is in an excited state of quantum number 2 n. It can emit a maximum energy photon of 204 eV. If it makes a transition to quantum state n, a photon of energy 40,8 eV is emitted. Find n. Z and the ground state energy (in eV) for this atom. Also, calculate the minimum energy (in eV) that can be emitted by this atom during de-excitation Ground state energy of hydrogen atom is 13.6 eV.
Ans.

111.When a beam of 10.6 eV photon of intensity 2 W/ m² falls on a platinum surface of area 1 x 10⁴ m²and work function 5.6 ev, 0.53% of the incident photons eject photo electrons. Find the number of photo electrons emitted per sec and their minimum and maximum energies in eV. [JEE’ 2000]
Ans.
“>

112.A hydrogen like atom (described by the Bohr model) is observed to emit six wavelengths, originating from all possible transition between a group of levels. These levels have energies between – 0.85 eV and – 0.544 eV (including both these values) Find the atomic number of the atom.Calculate the smallest wavelength emitted in these transitions.    [JEE’ 2002]
Ans.

113.Two metallic plates A and B each of area 5 x lO”⁴ m², are placed at a separation of 1 cm. Plate B carries a positive charge of 33.7 x 10~¹² A monochromatic beam of light, with photons of energy 5 eV each, starts falling on plate A at t = 0 so that 10¹⁶ photons fall on it per square meter per second. Assume that one photo electron is emitted for every 10⁶ incident photons. Also assume that all the emitted photo electrons are collected by plate B and the work function of plate A remains constant at the value 2 eV. Determine [JEE’ 2002]
a)the number of photo electrons emitted up to t = 10 sec.
b)the magnitude of the electric field between the plates A and B at t = 10 s and
c)the kinetic energy of the most energetic  photo electron emitted at t = 10 s when it reaches plate B.
(Neglect the time taken by photo electron to reach plate B)
Ans.

Ans.

Ans.

Ans.

117.The photons from the Balmer series in Hydrogen spectrum, having wavelengths between 450 nm to 700 nm, are incident on a metal surface of work function 2 eV. Find the maximum kinetic energy of ejected electron. (Given he = 1242 e V nm) [2004-4 Marks]
Ans.

“>

Ans.

119.Highly energetic electrons are bombarded on a target of an element containing 30 neutrons. The ratio of radii of nucleus to that of helium nucleus is (14)^1/3. Find          [JEE 2005]
a)atomic number of the nucleus
b)the frequency of K_a line of the X-ray produced. (R = l.lx 10⁷ nr¹ and c = 3 x 10⁸ m/s)
Ans.

120.In hydrogen-like atom (z = 11), n^th line of Lyman series has wavelength 1 equal to the de-Broglie’s wavelength of electron in the level from which it originated. What is the value of n? [JEE 2006]
Ans.

Ans.

122.Light from a dischargebe containing hydrogen atoms falls on the surface of a piece of sodium. The kinetic energy of the fastest photoelectrons emitted from sodium is 0.73 eV. The work function for sodium is 1.82 eV. Find –
a)the energy of the photons causing the photoelectric emission.
b)the quantum numbers of the two levels involved in the emission of these photons.
c)the change in the angular momentum of the electron in the hydrogen atom in the above transition, and
d)the recoil speed of the emitting atom assuming it to be at rest before the transition.
(Ionization potential of hydrogen is 13.6 eV)[1992-lO Marks]
Ans.

123.A neutron of kinetic energy 65 eV collides inelastically with a singly ionized helium atom at rest . It is scattered at an angle of 90° with respect of its original direction.
i)Find the allowed values of the energy of the neutron & that of the atom after collision.
ii)If the atom gets de-excited subsequently by emitting radiation , find the frequencies of the emitted radiation. (Given : Mass of he atom = 4x(mass of neutron), ionization energy of H atom =13.6 eV)     [JEE ’93]
Ans.




“>

124.A small quantity of solution containing ²⁴Na  radionuclide (half life 15 hours) of activity 1.0 microcurie is injected into the blood of a person. A sample of the blood of volume 1 cm³ taken after 5 hours shows an activity of 296 disintegrations per minute. Determine the total volume of blood in the body of the person. Assume that the radioactive solution mixes uniformly in the blood of the personal Curie = 3.7 x 10¹⁰ disintegrations per second) [JEE’94]
Ans.

125.A hydrogen like atom (atomic number Z) is in a higher excited state of quantum number n. This excited atom can make a transition to the first excited state by successively emitting two photons of energies 10.20 eV & 17.00 eV respectively. Alternatively , the atom from the same excited state can make a transition to the second excited state by successively emitting two photons of energies 4.25 eV & 5.95 eV respectively. Determine the values of n & Z. (Ionisation energy of hydrogen atom = 13.6 eV)           [JEE’94]
Ans.
“>

126.An electron, in a hydrogen-like atom, is in an excited state. It has a total energy of -3.4 eV. Calculate (i) the kinetic energy and (ii) the de Broglie wavelength of the electron. [1996-3Marks]
Ans.

127.At a given instant there are 25% undecyaed radio-active nuclei in a sample. After 10 second, the number of undecayed nuclei reduces to 12.5%. Calculate
(i) mean-life of the nuclei, and
(ii) the time in which the number of undecayed nuclei will further reduced to 6.25% of the reduce number.         [1996-3Marks]
Ans.

128.In an ore containing uranium; the ratio of U²³⁸ to Pb²⁰⁶ nuclei is 3. Calculate the age of the ore, assuming that all the lead present in the ore is the final stable product of U²³⁸. Take the half-life of U²³⁸ to be 4.5 x 10⁹ year.        [1997-5Marks]
Ans.

Ans.

Ans.

Ans.

132.In a nuclear reaction 235 U undergoes fission liberating 200 Me V of energy . The reactor has a 10% efficiency and produces 1000 MW power, if the reactor is to function for 10 year, find the total¹ mass of uranium required. [2001-5 Marks]
Ans.

Ans.

Ans.

Ans.

Ans.

Ans.

Ans.

139.A silver sphere of radius 1 cm and work funtion 4.7 eV is suspended from an insulting thread in free-space. It is under continuous illumination of 200 nm wavelength light. As photo electrons are emitted, the sphere gets charged and acquires a potential. The maximum number of photo elec­trons emitted from the sphere is A x 10^z (Where 1< A <10). The value of “Z” is (2011)
Ans.

True / False

140.The kinetic energy of photo electrons emitted by a photosensitive surface depends on the intensity of the incident radiation.          [1981-2 Marks]
Ans.

141.In a photoelectric emission process the maximum energy, of the photo electrons increases with increasing intensity of the incident light. [1986-3 Marks]
Ans.

False : Maximum energy of photoelectrons increases with increasing frequency of incident light. The number of photoelectrons emitted increases with increasing intensity of light.

142.The order of magnitude of the density of nuclear matter is 10⁴ kg m^-2  [1989-2 Marks]
Ans.
False : Nuclear density is of the order of I0ⁱ⁷ kg/m³. Hence the statement is false.

Fill in the blanks

143.To produce characteristic X-rays using a tungsten target in an X-ray generator, the accelerating voltage should be greater than………. volt and the energy of the characteristic radiation is……….. eV.(The binding energy of the innermost electron in tungsten is 40 keV.)  [1983-2 Marks]
Ans.

144.The radioactive decay rate of a radioactive element is found to be 10³ disintegration/second at a certain time. If the half life of the element is one second, the decay rate after one second is…………. and after three second is………..     [1983-2 Marks]
Ans.

145.The maximum kinetic energy of electrons emitted in the photoelectric effect is linearly dependent on the…………. of the incident radiation  . [1984-2Marks]
Ans.

146.In the uranium radioactive series, the initial nucleus is U and the final nucleus is Pb . When the uranium nucleus decays to lead, the number of a -particles emitted are……… and the number of (3 –

particles emitted are………            [1985-2Marks]
Ans.

147.When the number of electrons striking the anode of an X-ray tube is increased the……. of the emitted X-rays increase, while when the speed of the electrons striking the anode are increased the cut-off wavelength of the emitted X-rays………………. [1986-2Marks]
Ans.

148.

Ans.

149. Atoms having the same…………….but different………. are called isotopes.  [1986-2 marks]
Ans.
Isotopes have same atomic number (Z) and different mass numbers (A).

150.

Ans.

151.

Ans.

152.

Ans.

153.In an X-ray tube, electrons accelerated through a potential differenc e of 15,000 volt strike a copper target. The speed of the emitted X-rays inside the tube is  ……..     rn/s.      [1992-1Mark]
Ans.

154.In the Bohr model of the hydrogen atom, the ratio of the kine tic energy to the total energy of the electron in a quantum state n is… [1992-lMark]
Ans.

Ans.

156.A potential difference of 20 KV is applied across an x-ray tube. The minimum wave length of X – rays generated is       [JEE’96]
Ans.

Ans.

158.

Ans.

159.The recoil speed of a hydrogen atom after it emits a photon in going from n = 5 state to n = 1 state is    m/s.      [1997-1Mark]
Ans.