AQA A-Level Physics: Medical physics (7408) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.10 Medical physics (3.10.1 to 3.10.6) of the AQA AS and A-level Physics (7407/7408) specification, version 1.4 (AS and A-level exams June 2016 onwards). The section is A-level only. Sections 3.9-3.13 are options: you study one, assessed in Paper 3 Section B.

Learn the content first with the study guide and the revision notes. Course pages: AQA A-level Physics hub, printable checklist and the free diagnostics.

Use: e = 1.60 × 10⁻¹⁹ C, h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹, I₀ = 1.0 × 10⁻¹² W m⁻². Use the “real is positive” sign convention.

Questions

1. Explain why the eye has better spatial resolution at the fovea in bright light than at the edge of the retina in dim light. [3]

2. A short-sighted person has a far point of 1.6 m and a near point of 0.15 m.

(a) Calculate the power of the lens that corrects the far point. [2] (b) Calculate the near point when this lens is worn. [2]

3.

(a) A long-sighted person has a near point of 1.2 m. Calculate the power of the lens needed to read at 0.25 m. [2] (b) A prescription for an astigmatic eye contains three numbers. State what each one specifies. [2]

4. Describe how a sound wave in air is transmitted to the inner ear and turned into nerve signals. [3]

5.

(a) Define intensity of a sound. [1] (b) A machine produces an intensity level of 92 dB at a worker’s ear. Calculate the intensity. [2] (c) A second, identical machine is switched on. Calculate the new intensity level. [2] (d) State why noise at work is measured in dBA rather than dB. [1]

6. Describe how the threshold-of-hearing curve of a person with noise-induced hearing loss differs from that of an older person with age-related loss, compared with a healthy young adult. [3]

7.

(a) Describe a normal ECG waveform, naming each feature and stating what happens in the heart at each. [3] (b) Explain why a gel is applied before the electrodes. [1]

8. Muscle has density 1075 kg m⁻³ and speed of sound 1590 m s⁻¹. Bone has Z = 7.80 × 10⁶ kg m⁻² s⁻¹; air has Z = 430 kg m⁻² s⁻¹.

(a) Calculate the acoustic impedance of muscle. [1] (b) Calculate the fraction of the intensity reflected at a muscle-bone boundary. [2] (c) On an A-scan, echoes from the front and back of a layer of muscle arrive 40 µs and 92 µs after the pulse. Calculate the thickness of the layer. [2] (d) Use a calculation to explain why gel is used between the probe and skin. Take skin to have the Z of muscle. [2]

9.

(a) Explain why a flexible endoscope needs both a coherent and a non-coherent fibre bundle. [3] (b) Describe how an MR scanner produces signals from a cross-section of a patient. [4]

10. An X-ray tube operates at 100 kV.

(a) Calculate the maximum photon energy in J. [2] (b) Calculate the minimum wavelength. [1] (c) Explain why the X-ray spectrum has both a continuous part and sharp lines. [3] (d) State the effect on the beam of (i) adding an aluminium filter, (ii) increasing the tube current. [2]

11. For the X-rays used, soft tissue has μ = 22 m⁻¹ and bone has μ = 110 m⁻¹ and density 1850 kg m⁻³.

(a) Calculate the half-value thickness of soft tissue. [1] (b) Calculate the fraction of the beam transmitted through 0.12 m of soft tissue. [2] (c) A second path is 0.10 m of soft tissue plus 0.02 m of bone. Calculate the fraction transmitted, and the ratio of the two transmitted intensities. [3] (d) Calculate the mass attenuation coefficient of bone. [1] (e) Explain why a barium meal is given before an X-ray of the stomach. [2] (f) Suggest why ultrasound, not CT, is used to check the growth of a fetus. [2]

12. A tracer has physical half-life 18 h and biological half-life 9.0 h. A patient is given 400 MBq.

(a) Calculate the effective half-life. [2] (b) Calculate the activity in the patient 24 h later. [2] (c) Explain the importance of the molybdenum-technetium generator. [2] (d) Describe how a gamma camera forms an image. [3] (e) Describe three ways of limiting the dose to healthy cells in high-energy X-ray therapy. [3]

Answers

1. At the fovea each cone has its own nerve connection [1]; so images on neighbouring cones give separate signals [1]; at the edge, in dim light, rods are used and many share one nerve fibre, so close images merge into one signal [1]. Examiner insight: The mark is for linking the nerve connections to resolution; “cones are better” with no mechanism gains little.

2. (a) u = ∞, v = −1.6 m, so P = 1/(−1.6) [1] = −0.63 D [1] (b) Image must be at v = −0.15 m: 1/u = −0.625 + 1/0.15 = 6.04 m⁻¹ [1]; u = 0.17 m [1] Examiner insight: A positive power in (a) loses the accuracy mark even if the number is right; ECF is usually allowed into (b).

3. (a) P = 1/0.25 + 1/(−1.2) = 4.00 − 0.83 [1] = +3.2 D [1] (b) Sphere: spherical power for overall focus [1]; cylinder power and axis: strength of the cylindrical correction and its orientation in degrees [1]. Examiner insight: For (a), a power with no sign or unit is likely to lose the accuracy mark.

4. Eardrum vibrates; ossicles lever the vibration to the oval window [1]; its smaller area increases the pressure on the cochlear fluid [1]; the basilar membrane vibrates and hair cells produce nerve impulses [1]. Examiner insight: Each step needs the right structure in the right order; vague “the ear amplifies it” earns nothing.

5. (a) Power per unit area normal to the direction of the wave [1]. (b) I = 10⁻¹² × 10^9.2 [1] = 1.6 × 10⁻³ W m⁻² [1] (c) Intensity doubles: 10 log(2 × 1.58 × 10⁹) [1] = 95 dB [1] (d) dBA weights the level for the ear’s frequency response, so it matches perceived loudness [1]. Examiner insight: In (c), adding 92 + 92 dB scores zero; you must add intensities.

6. Both curves are higher than the healthy curve where hearing is lost [1]; noise damage raises the threshold over a band of frequencies, giving a notch [1]; age raises it most at high frequencies and lowers the upper frequency limit [1]. Examiner insight: Comparative wording (“higher threshold at…”) is needed; describing only one person caps the marks.

7. (a) P wave: atria depolarise and contract [1]; QRS complex: ventricles depolarise and contract [1]; T wave: ventricles repolarise [1]. (b) It gives good electrical contact with low resistance between skin and electrode [1]. Examiner insight: Naming P, QRS and T without the heart event at each earns no credit.

8. (a) Z = 1075 × 1590 = 1.71 × 10⁶ kg m⁻² s⁻¹ [1] (b) ((7.80 − 1.71)/(7.80 + 1.71))² [1] = 0.41 [1] (c) d = c × Δt/2 = 1590 × 52 × 10⁻⁶/2 [1] = 0.041 m [1] (d) Air-skin: ((1.71 × 10⁶ − 430)/(1.71 × 10⁶ + 430))² = 0.999, almost all reflected [1]; gel has Z close to skin, so most ultrasound enters the body [1]. Examiner insight: In (c), use the speed in the layer itself and the time difference; forgetting to halve gives a common one-mark loss.

9. (a) Non-coherent bundle carries light in to illuminate the area [1]; coherent bundle has fibres in matching positions at both ends [1]; so it carries the image back without scrambling it [1]. (b) Strong field: protons align and precess about the field lines [1]; gradient coils select the slice [1]; short RF pulses excite protons in small regions in turn [1]; as they de-excite they emit RF signals, detected and processed by computer [1]. Examiner insight: Field production and relaxation times gain nothing here; spend the words on the listed steps.

10. (a) E = eV = 1.60 × 10⁻¹⁹ × 1.00 × 10⁵ [1] = 1.60 × 10⁻¹⁴ J [1] (b) λ = hc/E = 1.24 × 10⁻¹¹ m [1] (c) Electrons decelerate in the target, losing varying energy [1], giving photons of all energies up to eV [1]; lines arise when inner-shell vacancies in target atoms are filled, giving fixed photon energies [1]. (d) (i) Removes low-energy photons, so lower patient dose [1]; (ii) higher intensity, same photon energies [1]. Examiner insight: In (d)(ii) saying the photon energy rises loses the mark; current changes only the number of photons.

11. (a) x½ = ln 2/22 = 0.032 m [1] (b) I/I₀ = e^(−22 × 0.12) [1] = 0.071 [1] (c) e^(−(22 × 0.10 + 110 × 0.02)) = e^(−4.4) [1] = 0.012 [1]; ratio = 0.0714/0.0123 = 5.8 [1] (d) μm = 110/1850 = 0.059 m² kg⁻¹ [1] (e) Barium absorbs X-rays strongly [1]; so the stomach outline contrasts with the soft tissue around it [1]. (f) Ultrasound is non-ionising, so no risk to the fetus [1]; it is cheap and real-time, while CT gives a high dose [1]. Examiner insight: In (c), add the μx terms in the exponent; multiplying the two exponentials separately is fine, but adding the fractions is not.

12. (a) 1/T_E = 1/9.0 + 1/18 [1]; T_E = 6.0 h [1] (b) 24 h = 4 effective half-lives [1]; 400/2⁴ = 25 MBq [1] (c) Mo-99 has a longer half-life (about 66 h, against about 6 h) and decays to Tc-99m [1]; Tc-99m is eluted on site when needed, so the short-lived tracer is always available [1]. (d) Collimator passes only photons travelling along its holes [1]; scintillator crystal flashes for each photon, detected by photomultiplier tubes [1]; relative pulse sizes locate each flash and a computer builds the image [1]. (e) Any three: several beams from different directions crossing at the tumour (or rotation) [1]; beam shaped to the tumour [1]; shielding of other organs, or dose split into sessions [1]. Examiner insight: Using T_P instead of T_E in (b) gives a wrong answer with no accuracy credit; method marks need T_E.

Where marks are usually lost

• Sign errors in lens power from a positive v for a virtual image.
• Adding decibels instead of intensities.
• Missing the factor of 2 for echo depth, or using the wrong time.
• Forgetting to square in the reflection equation.
• Stating that tube current changes photon energy.
• Swapping the roles of coherent and non-coherent bundles.
• Using T_P where T_E is needed.
• Giving a single vague comparison point when resolution, convenience and safety are each worth credit.

Next steps

• Revision notes
• Study guide
• AQA A-level Physics hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA AS and A-level Physics (7407/7408) specification, version 1.4, July 2026 (AS and A-level exams June 2016 onwards), section 3.10 Medical physics.