AQA A Level Physics 3.10 Medical Physics (Option B) Exam Questions

About AQA A Level Physics 3.10 Medical Physics (Option B)

Medical Physics is optional topic 3.10 in AQA A Level Physics and is assessed in Section B of Paper 3 if your school chooses it. Sections 3.10.1 and 3.10.2 cover the physics of the eye (lenses, defects of vision, rods and cones, spatial resolution) and the ear (structure, sensitivity, intensity level in decibels and hearing loss). Sections 3.10.3 to 3.10.6 cover biological measurement with the ECG, non-ionising imaging using ultrasound and fibre-optic endoscopes, X-ray imaging including CT scanning, and radionuclide imaging and therapy with the gamma camera and PET. The option builds on refraction, waves and nuclear physics from Papers 1 and 2.

What you need to know

• 3.10.1 The eye Lens power P = 1/f in dioptres, and 1/u + 1/v = 1/f with the real is positive convention. Myopia (short sight) is corrected with a diverging lens and hypermetropia (long sight) with a converging lens; astigmatism needs a cylindrical lens. Rods give vision in dim light and cones give colour vision.
• 3.10.2 The ear Sound passes through the outer ear, the eardrum and ossicles act as an impedance matching system and the cochlea converts vibrations into nerve signals. Intensity level = 10 log(I/I0) dB, with I0 = 1.0 x 10^-12 W m-2. The dBA scale adjusts for the ear's frequency response, and equal loudness curves show the ear is most sensitive at a few kHz.
• 3.10.3 ECG Electrodes on the skin detect the potential changes as the heart depolarises and repolarises. Know the P wave, QRS complex and T wave and why good electrode contact is needed.
• 3.10.4 Ultrasound and endoscopy Piezoelectric transducers produce and detect ultrasound pulses. Acoustic impedance Z = rho c and the reflected fraction is (Z2 - Z1)^2/(Z2 + Z1)^2, so a coupling gel is used. Know A-scans and B-scans. Endoscopes use coherent and non-coherent fibre bundles.
• 3.10.5 X-ray imaging X-rays are produced at a rotating anode. Attenuation I = I0 e^(-mu x), with half-value thickness = ln 2/mu and mass attenuation coefficient mu/rho. Contrast media and intensifying screens improve images; CT scanning builds a 3D image from many angles at a higher dose.
• 3.10.6 Radionuclide imaging and therapy Technetium-99m is ideal for imaging because it emits gamma only and has a 6 hour half-life. Effective half-life: 1/TE = 1/TB + 1/TP. Know the gamma camera (collimator, scintillator, photomultiplier tubes), PET scanning and the use of high-energy X-rays and implants in therapy.

Common mistakes in 3.10 Medical Physics (Option B)

• Saying a myopic eye is corrected with a converging lens. Short sight needs a diverging lens so that distant objects focus on the retina.
• Treating intensity level as proportional to intensity. Doubling intensity adds only about 3 dB because the scale is logarithmic.
• Forgetting why coupling gel is needed. Without it almost all ultrasound reflects at the air-skin boundary because the acoustic impedances are so different.
• Adding biological and physical half-lives to get the effective half-life. Add their reciprocals; the effective half-life is shorter than both.
• Describing a gamma camera collimator as focusing the gamma rays. It only lets through gamma rays travelling parallel to the tubes so the image position is correct.

Try three 3.10 Medical Physics (Option B) exam-style questions