STP Physical Sciences 2 — Questions and Answers
Question 1: Which type of electromagnetic radiation has the shortest wavelength used in medical imaging?
- Infrared
- Microwave
- X-ray (Correct answer)
- Ultraviolet
Correct answer: X-ray
X-rays have wavelengths in the range of 0.01–10 nm, making them the shortest wavelength radiation routinely used in medical imaging.
Question 2: What is the half-life of a radioactive isotope if 75% of a sample decays in 20 years?
- 5 years
- 10 years (Correct answer)
- 15 years
- 20 years
Correct answer: 10 years
If 75% decays, 25% remains; since (1/2)^2 = 0.25, two half-lives have passed, so the half-life is 20/2 = 10 years.
Question 3: In a semiconductor detector for ionising radiation, what charge carriers are collected?
- Protons and neutrons
- Electrons and holes (Correct answer)
- Positrons and electrons
- Ions and molecules
Correct answer: Electrons and holes
Ionising radiation creates electron-hole pairs in the semiconductor, and these charge carriers are swept to electrodes by an applied electric field.
Question 4: What physical principle underlies Magnetic Resonance Imaging (MRI)?
- Photoelectric absorption of X-rays
- Nuclear spin precession in a magnetic field (Correct answer)
- Compton scattering of gamma rays
- Ultrasonic reflection at tissue boundaries
Correct answer: Nuclear spin precession in a magnetic field
MRI exploits the precession of hydrogen nuclei (protons) aligned in a strong magnetic field when perturbed by a radiofrequency pulse.
Question 5: Which law states that the pressure of a fixed mass of gas is inversely proportional to its volume at constant temperature?
- Charles's Law
- Gay-Lussac's Law
- Boyle's Law (Correct answer)
- Dalton's Law
Correct answer: Boyle's Law
Boyle's Law states that P ∝ 1/V at constant temperature, meaning pressure and volume are inversely proportional for an ideal gas.
Question 6: What is the unit of absorbed radiation dose in the SI system?
- Sievert (Sv)
- Becquerel (Bq)
- Gray (Gy) (Correct answer)
- Roentgen (R)
Correct answer: Gray (Gy)
The Gray (Gy) is the SI unit of absorbed dose, defined as 1 joule of energy deposited per kilogram of tissue.
Question 7: A photon travels through a medium with refractive index 1.5. What is its speed in that medium given that c = 3×10⁸ m/s?
- 4.5×10⁸ m/s
- 3×10⁸ m/s
- 2×10⁸ m/s (Correct answer)
- 1.5×10⁸ m/s
Correct answer: 2×10⁸ m/s
Speed in medium = c/n = 3×10⁸ / 1.5 = 2×10⁸ m/s.
STP Physical Sciences 2
Which type of electromagnetic radiation has the shortest wavelength used in medical imaging?