AQA A Level Physics Paper 3 Section B: Medical physics June 2018 question paper 7408/3BB

An endoscope is used to view an area inside the body. The endoscope contains two bundles of optical fibres.

Name each bundle and explain its use in the process.

A single optical fibre is placed in air. The optical fibre has a core surrounded by cladding. The critical angle is 75° at the core-cladding boundary.

Complete Figure 1 to show how the refractive index varies with radial distance from the centre of the core to the air surrounding the fibre. Your answer should be supported by a suitable calculation.

refractive index of core = 1.6

The fovea in a typical human eye consists of cones which have an average diameter

An eye looks directly at two point sources of light which are 12 mm apart at a distance of 61 m from the centre of the eye lens. The fovea is at the centre of the retina a distance of 21 mm behind the centre of the eye lens.

Deduce whether the eye would be able to resolve the two images formed at the fovea.

Three types of cone are present at the fovea.

On Figure 2 sketch and clearly label three curves to show how the relative light absorption of each type of cone varies with wavelength.

In the past, doctors could only use a simple X-ray image to assess head injuries. A CT scan is now a preferred technique.

Discuss why the CT scan has replaced the simple X-ray image to assess head injuries, but a simple X-ray procedure is suitable for assessing other injuries.

In your answer, you should:

• describe the basic principles of a CT scanner

• discuss the advantage of the CT scan over a simple X-ray image for head injuries

• explain why a simple X-ray procedure is more suitable for assessing other injuries.

A source of constant output power is used to generate a sound which is measured using a sound meter.

The dashed line in Figure 3 shows the intensity level curve over a range of frequencies with the meter set to the dB setting.

A student sketches a curve, over the same frequency range, which he thinks would be obtained when the meter is changed to the dBA setting. The curve drawn by the student is shown as the solid line in Figure 3.

Discuss whether the dBA curve drawn is correct.

Mesh barriers are set up to keep pedestrians at a safe distance from a noisy drill. The maximum noise level which pedestrians should be subjected to is 110 dB The drill emits sound with a power of 7.8 W and acts as a point source. The mesh barriers are set up a distance of 2.0 m from the drill.

Discuss whether this will keep pedestrians at a safe distance from the sound source.

High-energy X-rays are used in the treatment of a cancer tumour inside a patient’s body. The patient is given a series of scans before the treatment is started.

Discuss how these scans are used to help provide the best and safest treatment for the patient when using the high-energy X-rays.

Lead is commonly used as shielding when using X-rays due to its small half-value thickness.

Which statement gives the correct meaning of half-value thickness? Tick (✓) the correct answer.

The half-value thickness of lead for 500 keV X-rays is 4.2 × 10 −3 m

Calculate the mass attenuation coefficient of lead for 500 keV X-rays.

State an appropriate unit for your answer.

mass attenuation coefficient =