CSS Physics chapter 3: Modern physics MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
64 exam-style questions on Modern physics, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A particle is moving so that its total energy is exactly twice its rest energy. Its kinetic energy is
A
equal to twice its rest energy
B
equal to its rest energy
C
half its rest energy
D
zero
Show answer
Answer: B. Total energy = rest energy + kinetic energy, so E = 2mc2 means KE = 2mc2 − mc2 = mc2.
2. Light of photon energy 4.5 eV falls on a metal whose work function is 2.0 eV. The maximum kinetic energy of the photoelectrons is
A
6.5 eV
B
2.5 eV
C
2.0 eV
D
4.5 eV
Show answer
Answer: B. Einstein's equation: Kmax = hf − φ = 4.5 − 2.0 = 2.5 eV. The stopping potential is therefore 2.5 V.
3. Taking c = 3 × 108 m s−1, the energy equivalent of a mass of 1 mg (10−6 kg) is
A
9 × 1013 J
B
9 × 107 J
C
9 × 1010 J
D
3 × 102 J
Show answer
Answer: C. E = mc2 = 10−6 × (3 × 108)2 = 10−6 × 9 × 1016 = 9 × 1010 J. The value 9 × 1013 J is the answer for 1 g.
4. In the Schrödinger description of an electron in an atom, the quantity |ψ|2 at a point represents
A
the probability per unit volume of finding the electron there
B
the energy of the electron at that point
C
the momentum of the electron at that point
D
the charge of the electron at that point
Show answer
Answer: A. ψ itself has no direct physical meaning; the Born rule interprets |ψ|2 as a probability density, so its integral over all space is 1.
5. In Compton scattering of X-rays by free electrons, the scattered X-ray photons have
A
a shorter wavelength, the decrease depending on the scattering angle
B
the same wavelength as the incident photons
C
a longer wavelength, the increase depending on the scattering angle
D
a longer wavelength, the same for every scattering angle
Show answer
Answer: C. The photon gives some energy to the recoiling electron, so it emerges with lower energy and longer wavelength; Δλ = (h/mec)(1 − cos θ) increases with the scattering angle.
6. In a common-emitter transistor amplifier the output voltage is
A
180° out of phase with the input voltage
B
in phase with the input voltage
C
90° ahead of the input voltage
D
always zero
Show answer
Answer: A. A rise of the base voltage raises the collector current, which increases the voltage drop across the collector resistor and lowers the collector voltage, so the output is inverted.
7. The visible lines of the hydrogen spectrum, known as the Balmer series, are produced by electron transitions that end on the level
A
n = 2
B
n = 1
C
n = 3
D
n = 4
Show answer
Answer: A. Transitions ending at n = 1 give the ultraviolet Lyman series, those ending at n = 3 the infrared Paschen series, and those ending at n = 2 the Balmer series.
8. A material object with non-zero rest mass cannot be accelerated to the speed of light because
A
its mass would fall to zero
B
its length would become infinite
C
its energy would have to become infinite
D
its temperature would become infinite
Show answer
Answer: C. As v approaches c, the total energy γmc2 grows without limit, so an infinite amount of work would be needed.
9. A sample contains 80 g of a radioactive isotope of half-life 3 days. The mass of the isotope remaining after 9 days is
A
26.7 g
B
20 g
C
40 g
D
10 g
Show answer
Answer: D. 9 days is three half-lives, so the mass is 80 × (½)3 = 10 g. Dividing 80 by 3 (26.7 g) wrongly treats the decay as linear.
10. In the Bohr model of hydrogen the energy of the level n is −13.6/n2 eV. The energy of the level n = 2 is
A
−3.4 eV
B
−6.8 eV
C
−1.5 eV
D
−0.85 eV
Show answer
Answer: A. E2 = −13.6/4 = −3.4 eV. The value −6.8 eV divides by n rather than n2.
64, each with the answer and a short explanation, across 4 topics: Special relativity, Quantum physics and atomic structure, Nuclear and particle physics, Solid-state physics and electronics. Every chapter test draws ten of them, so you can take it many times.
Is the CSS Physics chapter 3 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
