Modern Physics — GK Questions with Answers

1. Cathode rays are made of which particles?
   1. protons
   2. electrons
   3. neutrons
   4. photons
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   Answer: (B) electrons

   Cathode rays are a stream of negatively charged electrons; their e/m came out the same for every gas and cathode.

2. Who first measured the charge of the electron?
   1. J. J. Thomson
   2. Chadwick
   3. Rutherford
   4. Robert Millikan
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   Answer: (D) Robert Millikan

   Millikan measured e = 1.6 × 10⁻¹⁹ C with the oil-drop experiment in 1909; Thomson had measured e/m.

3. Who discovered the neutron?
   1. Chadwick
   2. Rutherford
   3. Bohr
   4. Goldstein
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   Answer: (A) Chadwick

   James Chadwick discovered the neutron in 1932 by firing α-particles at beryllium (Nobel 1935).

4. In Rutherford’s gold-foil experiment most α-particles went straight through. What did this show?
   1. the nucleus is negative
   2. electrons are heavy
   3. most of the atom is empty
   4. the atom is indivisible
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   Answer: (C) most of the atom is empty

   Most particles going straight shows that the atom has a lot of empty space; very few bouncing back is proof of a small heavy nucleus.

5. In Bohr’s model what is the energy of the ground state of hydrogen?
   1. −3.4 eV
   2. +13.6 eV
   3. −13.6 eV
   4. 0 eV
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   Answer: (C) −13.6 eV

   Eₙ = −13.6/n² eV; at n = 1 it is −13.6 eV, so the ionisation energy is 13.6 eV.

6. Which series of the hydrogen spectrum is in the visible region?
   1. Balmer
   2. Lyman
   3. Paschen
   4. Pfund
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   Answer: (A) Balmer

   The Balmer series (n₁ = 2) is in the visible region; Lyman in the ultraviolet and Paschen-Brackett-Pfund in the infrared.

7. What is the same in isotopes?
   1. mass number
   2. neutron number
   3. mass of the nucleus
   4. atomic number
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   Answer: (D) atomic number

   Isotopes have equal protons (Z) and different neutrons, so their chemical properties stay the same.

8. What are ¹⁴C and ¹⁴N to each other?
   1. isotopes
   2. isobars
   3. isotones
   4. isoelectronic
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   Answer: (B) isobars

   Both have mass number 14 but different atomic numbers (6 and 7) — so they are isobars.

9. 1 atomic mass unit (u) is equivalent to how much energy?
   1. 931.5 MeV
   2. 9.31 MeV
   3. 1 MeV
   4. 200 MeV
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   Answer: (A) 931.5 MeV

   From E = mc², 1 u = 931.5 MeV; 200 MeV is the energy per fission.

10. Binding energy per nucleon is highest near which nucleus?
    1. hydrogen
    2. helium
    3. iron
    4. uranium
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    Answer: (C) iron

    BE/A ≈ 8.8 MeV is highest near iron-56 — so heavy nuclei give energy by breaking and light ones by joining.

11. Who discovered radioactivity?
    1. Marie Curie
    2. Henri Becquerel
    3. Röntgen
    4. Rutherford
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    Answer: (B) Henri Becquerel

    Henri Becquerel discovered radioactivity in uranium salts in 1896; the Curies discovered polonium and radium.

12. What change happens to the atomic and mass numbers in α-decay?
    1. Z + 1, A same
    2. no change
    3. Z − 1, A same
    4. Z − 2, A − 4
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    Answer: (D) Z − 2, A − 4

    An α-particle is a helium nucleus (2 protons, 2 neutrons), so Z falls by two and A by four.

13. Which radiation has the highest penetrating power?
    1. α
    2. β
    3. all equal
    4. γ
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    Answer: (D) γ

    γ rays are uncharged electromagnetic waves — several cm of lead are needed to stop them; paper stops α.

14. The half-life of a substance is 5 days. What fraction is left after 20 days?
    1. 1/4
    2. 1/16
    3. 1/8
    4. 1/32
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    Answer: (B) 1/16

    20 days = 4 half-lives; fraction left (½)⁴ = 1/16.

15. Which isotope is used in carbon dating?
    1. C-12
    2. C-13
    3. C-14
    4. U-238
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    Answer: (C) C-14

    C-14 (half-life ≈ 5730 years) gives the age of biological objects up to about 50,000 years.

16. Which isotope is commonly used in radiotherapy of cancer?
    1. cobalt-60
    2. sodium-24
    3. carbon-14
    4. iodine-131
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    Answer: (A) cobalt-60

    Gamma rays from cobalt-60 destroy cancer cells; I-131 is for the thyroid.

17. What is the work of a moderator in a nuclear reactor?
    1. absorbing neutrons
    2. slowing fast neutrons
    3. carrying heat out
    4. stopping radiation
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    Answer: (B) slowing fast neutrons

    The moderator (heavy water, graphite) slows neutrons so that they can split U-235; absorbing is the job of control rods.

18. Control rods in a nuclear reactor are made of —
    1. graphite
    2. heavy water
    3. uranium
    4. cadmium or boron
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    Answer: (D) cadmium or boron

    Cadmium and boron absorb neutrons well; pushing the rods in slows the reaction.

19. If the multiplication factor k = 1 in a chain reaction —
    1. controlled steady reaction (reactor)
    2. the reaction dies out
    3. explosion (bomb)
    4. nothing happens
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    Answer: (A) controlled steady reaction (reactor)

    k = 1 is the critical state — equal neutrons in every generation, as needed in a reactor; at k > 1 a bomb.

20. Which was India’s first nuclear research reactor (also the first in Asia)?
    1. Dhruva
    2. CIRUS
    3. Apsara
    4. Tarapur
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    Answer: (C) Apsara

    Apsara started at Trombay on 4 August 1956; Tarapur (1969) was the first nuclear power station.

21. What is the source of the Sun’s energy?
    1. nuclear fission
    2. chemical burning
    3. nuclear fusion
    4. radioactive decay
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    Answer: (C) nuclear fusion

    In the Sun’s core at ≈ 1.5 crore K hydrogen nuclei join to form helium — this is fusion.

22. The hydrogen bomb is based on which principle?
    1. fusion
    2. fission
    3. chemical explosion
    4. radioactive decay
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    Answer: (A) fusion

    A hydrogen bomb has uncontrolled fusion; the temperature needed to start it comes from an atom (fission) bomb inside.

23. When was India’s first nuclear test "Smiling Buddha"?
    1. 4 August 1956
    2. 11 May 1998
    3. 6 August 1945
    4. 18 May 1974
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    Answer: (D) 18 May 1974

    The first test at Pokhran (Jaisalmer) was on 18 May 1974; Pokhran-II was on 11 and 13 May 1998.

24. The maximum kinetic energy of photoelectrons depends on —
    1. intensity
    2. frequency
    3. time of exposure
    4. size of the metal
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    Answer: (B) frequency

    Kmax = hν − φ — only on frequency (and the work function of the metal); intensity only raises the number of electrons.

25. For what work did Einstein get the Nobel Prize?
    1. photoelectric effect
    2. theory of relativity
    3. E = mc²
    4. Brownian motion
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    Answer: (A) photoelectric effect

    The 1921 physics Nobel was for the discovery of the law of the photoelectric effect.

26. Light of 400 nm falls on a metal of work function 2.75 eV. Kmax is about?
    1. 2.75 eV
    2. 3.10 eV
    3. 0.35 eV
    4. 5.85 eV
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    Answer: (C) 0.35 eV

    Photon energy = 1240/400 = 3.10 eV; Kmax = 3.10 − 2.75 = 0.35 eV.

27. What is the formula of the de Broglie wavelength?
    1. λ = hν
    2. λ = h/mv
    3. λ = mc²
    4. λ = c/h
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    Answer: (B) λ = h/mv

    Every moving particle has a wave λ = h/p = h/mv linked with it; Davisson-Germer confirmed it in 1927.

28. Who discovered X-rays?
    1. Marie Curie
    2. Thomson
    3. Becquerel
    4. Röntgen
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    Answer: (D) Röntgen

    Röntgen discovered X-rays in 1895 and got the first physics Nobel in 1901.

29. Which is NOT a property of laser light?
    1. monochromatic
    2. coherent
    3. intense
    4. spreading in all directions
    Show answer

    Answer: (D) spreading in all directions

    A laser is highly directional — it spreads very little; the other three are its properties.

30. What is added to silicon to make an n-type semiconductor?
    1. boron
    2. phosphorus
    3. aluminium
    4. gallium
    Show answer

    Answer: (B) phosphorus

    Pentavalent phosphorus (or As, Sb) gives an extra electron; boron, Al, Ga are trivalent and make p-type.

31. A p-n junction diode is in forward bias when —
    1. p is joined to negative and n to positive
    2. both to positive
    3. p is joined to positive and n to negative
    4. there is no battery
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    Answer: (C) p is joined to positive and n to negative

    Joining p to the positive and n to the negative terminal makes the depletion layer thin and current flows.

32. Which diode is used as a voltage regulator?
    1. Zener diode
    2. photodiode
    3. LED
    4. solar cell
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    Answer: (A) Zener diode

    A Zener diode works in the reverse breakdown region and keeps the voltage across it steady even when the current changes.

33. Which logic gates are called "universal"?
    1. AND and OR
    2. NAND and NOR
    3. NOT and XOR
    4. OR and NOT
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    Answer: (B) NAND and NOR

    All other gates can be made by joining only NAND gates (or only NOR gates).

34. When is the output of an AND gate 1?
    1. when any one input is 1
    2. when the inputs differ
    3. when both inputs are 0
    4. when both inputs are 1
    Show answer

    Answer: (D) when both inputs are 1

    An AND gate gives 1 only when all inputs are 1; 1 when any one is 1 is the rule of OR.

35. TV and mobile signals (above 30 MHz) travel as —
    1. space waves (line of sight)
    2. sky waves
    3. ground waves
    4. sound waves
    Show answer

    Answer: (A) space waves (line of sight)

    Waves above 30 MHz pass through the ionosphere, so they are sent by direct line of sight or by satellite.

36. The Chandrasekhar limit (≈ 1.44 solar masses) is related to —
    1. orbits of planets
    2. the size of a black hole
    3. the maximum mass of a white dwarf star
    4. the diameter of a galaxy
    Show answer

    Answer: (C) the maximum mass of a white dwarf star

    A core heavier than this cannot stay a white dwarf and becomes a neutron star or black hole; Chandrasekhar got the 1983 Nobel for it.

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