AQA A Level Physics Paper 3 Section B: Medical physics June 2019 question paper 7408/3BB

Car drivers must be able to

• read a speedometer from a distance of 50 cm

• read a number plate from a distance of 20.5 m.

A driver has an unaided far point of 55 cm and an unaided near point of 25 cm.

Identify the driver’s eye defect.

Figure 1 shows the position of a number plate at a distance of 20.5 m in front of the driver’s unaided eye.

Figure 2 shows the same situation and the position of a corrective lens.

Complete both ray diagrams to show how and where the image of the number plate is formed in each case. Add a suitable lens to Figure 2.

An optician considers the use of three different lenses, A, B and C, for use by the driver when driving.

Power of A = ─2.18D Power of B = ─1.77D Power of C = +1.95D

Deduce which lens is suitable. Support your answer with calculations.

Three customers, P, Q and R, are sitting in a café listening to music from a loudspeaker. Customer P is 11 m from the loudspeaker. At the position of customer P, the sound

Customer P moves to a distance of 7.0 m from the loudspeaker.

Calculate the sound intensity at the new position of customer P. Assume that the loudspeaker is a point source.

The sound intensity level is 65 dB at the position of customer Q and 42 dB at the position of customer R.

Customer Q perceives the loudness of the sound differently to customer R.

Discuss whether the use of intensity level or intensity is more appropriate to compare the perceived loudness.

Customers P, Q and R move to the same distance from the loudspeaker.

Customer P is 80 years old and has hearing loss due to her age. Customer Q is 35 years old and has hearing loss due to working in an extremely noisy environment. Customer R is 35 years old and has no hearing loss.

The hearing defects of P and Q affect their perception of the music being played.

Describe how their perceptions are different from that of R.

Figure 3 shows the X-ray spectrum produced in a medical X-ray machine at a particular anode potential difference (pd).

In an X-ray tube, electrons collide with a tungsten target.

Explain how the continuous spectrum and the characteristic spectra are produced by these electron collisions.

Continuous spectrum

Characteristic spectra

The dashed line on Figure 4 shows the X-ray spectrum for the initial anode pd.

Sketch on Figure 4 the X-ray spectrum produced when the anode pd is increased.

In the medical X-ray machine, the X-rays produced with the initial anode pd are now passed through an aluminium filter. The dashed line on Figure 5 shows the X-ray spectrum for the initial anode pd.

Sketch on Figure 5 the X-ray spectrum of the X-rays that emerge from the filter.

Ultrasound is commonly used in medical procedures.

An ultrasound A-scan is used to find the length l of an eye as shown in Figure 6. Figure 7 shows the simplified A-scan for the eye. A short pulse of ultrasound is transmitted at time t = 0

The average speed of ultrasound in the eye = 1560 m s¹.

Calculate l.

Amniocentesis is a procedure where a tube is inserted into a uterus to remove some cells and fluid from around a foetus. For the procedure to be carried out safely the positions of the needle, foetus and placenta must be determined accurately.

Discuss whether an A-scan or a B-scan should be used for amniocentesis.

In your answer, you should:

• outline the differences between an A-scan and a B-scan

• describe the advantages and disadvantages of each type of scan

• explain why your chosen scan should be used for this procedure.