Modern Physics: Relativity and Quantum — High School Physics practice questions

According to E = hf, a photon with higher frequency has:

1. more energy
2. less energy
3. the same energy
4. a longer wavelength

Answer: A — more energy

A) Correct — Energy is proportional to frequency. B) Reverses the relationship. C) Energy depends on frequency. D) Higher frequency means shorter wavelength.

The Sun produces energy mainly by:

1. nuclear fission
2. nuclear fusion
3. burning hydrogen gas chemically
4. radioactive decay of uranium

Answer: B — nuclear fusion

B) Correct — Hydrogen nuclei combine to form helium. A) Splits heavy nuclei, as in power plants. C) Is a chemical process. D) Isn't the Sun's main source.

A sample of 80 g has a half-life of 2 days. How much remains after 6 days?

1. 20 g
2. 26.7 g
3. 10 g
4. 0 g

Answer: C — 10 g

C) Correct — Three half-lives: 80 → 40 → 20 → 10. A) Uses two half-lives. B) Divides by 3. D) Assumes it all decays.

Einstein's equation E = mc² shows that:

1. energy always equals mass
2. light has a large mass
3. mass increases with temperature only
4. mass can be converted into energy

Answer: D — mass can be converted into energy

D) Correct — A small mass releases enormous energy. A) The c² factor matters. B) Photons have no rest mass. C) Is not its meaning.

In the Bohr model, an atom emits light when an electron:

1. jumps to a higher energy level
2. drops to a lower energy level
3. leaves the nucleus
4. stays in the same level

Answer: B — drops to a lower energy level

B) Correct — The photon carries away the energy difference. A) Absorbs light instead. C) Electrons aren't in the nucleus. D) No energy change occurs.

Gamma rays are:

1. helium nuclei
2. high-energy electromagnetic radiation
3. fast electrons
4. slow neutrons

Answer: B — high-energy electromagnetic radiation

B) Correct — They have no mass or charge. A) Are alpha particles. C) Are beta particles. D) Are not gamma rays.

A key postulate of special relativity is that:

1. light speeds up when its source moves toward you
2. time passes the same for everyone
3. light's speed in a vacuum is the same for all observers
4. objects can exceed light speed

Answer: C — light's speed in a vacuum is the same for all observers

C) Correct — It doesn't depend on the source's motion. A) Light's speed doesn't change. B) Time dilation shows otherwise. D) Objects with mass can't.

An atom's nucleus contains:

1. electrons and protons
2. only electrons
3. protons and neutrons
4. only neutrons

Answer: C — protons and neutrons

C) Correct — Electrons surround the nucleus. A) Electrons are outside the nucleus. B) Electrons are outside the nucleus. D) Protons are also in the nucleus.

Electrons passing through a crystal produce a diffraction pattern. This shows that electrons:

1. have wave properties
2. have no mass
3. are only particles
4. travel at light speed

Answer: A — have wave properties

A) Correct — This is wave-particle duality. B) Electrons have mass. C) Diffraction is a wave behavior. D) They don't.

In beta-minus decay, a neutron changes into:

1. a proton and a neutron
2. an alpha particle
3. a proton and an electron
4. two electrons

Answer: C — a proton and an electron

C) Correct — The atomic number increases by 1. A) Doesn't conserve charge. B) Is a different decay. D) Doesn't conserve charge.

Why do atoms emit line spectra?

Electrons occupy only certain energy levels (the Bohr model). When an electron drops to a lower level it emits a photon whose energy equals the difference, E = hf, so each element emits only specific wavelengths.

What is half-life?

The time for half of the radioactive nuclei in a sample to decay. After n half-lives, (1/2)ⁿ of the original remains; after 3 half-lives, 1/8 is left.

What is time dilation?

A result of special relativity: a moving clock runs slow compared with one at rest relative to the observer, by the factor γ = 1/√(1 − v²/c²). It is measured in fast-moving muons and must be corrected for in GPS satellite clocks.

What is the energy of a photon?

E = hf = hc/λ, where h = 6.63 × 10⁻³⁴ J·s (Planck's constant).