1True or false
Microwaves from a mobile phone can ionise atoms in the brain if the phone is used for long enough.
Show answer
Answer: False
Ionisation depends on the energy of each photon; a microwave photon has far too little, however many arrive or however long they last. Strong microwaves can only heat tissue.
!Common mistakeLearners think a long exposure adds up to ionisation; each photon must be energetic enough on its own.
2True or false · ★ Challenge
Keyhole surgery using an endoscope exposes the patient to ionising radiation.
Show answer
Answer: False
An endoscope uses visible light carried by optical fibres, which is non-ionising.
!Common mistakeLearners group all hospital imaging together as "radiation"; endoscopy, like ultrasound and MRI, gives no ionising dose.
3True or false · ★ Challenge
A lead apron gives the same protection against gamma rays from a cobalt-60 therapy source as it does against the X-rays used for a chest image.
Show answer
Answer: False
An apron has only about 0.5 mm of lead, enough to absorb most low-energy diagnostic X-rays, but cobalt-60 gamma rays (over 1 MeV) need several centimetres of lead.
!Common mistakeLearners think "lead stops radiation" whatever the energy; more energetic photons need much thicker shielding.
4True or false · ★ Challenge
A gamma emitter is preferred to an alpha emitter as a tracer for imaging because gamma rays leave the body and reach the camera, while alpha particles would be absorbed inside the body and only cause damage.
Show answer
Answer: True
Gamma rays are penetrating and weakly ionising; alpha particles travel only a fraction of a millimetre in tissue and deposit all their energy there.
!Common mistakeLearners sometimes think the most ionising radiation gives the clearest image; an image needs radiation that escapes the body.
5True or false
A very strong ultrasound echo shows that the two materials at a boundary have very different acoustic impedances.
Show answer
Answer: True
The fraction reflected depends on ((Z₂ − Z₁)/(Z₂ + Z₁))², so a big difference in impedance gives a strong reflection.
!Common mistakeLearners think echoes depend on density alone; it is the acoustic impedance Z = ρv that matters.
6Multiple choice · ★ Challenge
In MRI, most of the signal used to build the image comes from:
- AHydrogen nuclei (protons) in water and fat
- BElectrons in the calcium of the bones
- CIron atoms in the red blood cells
- DRadioactive tracers injected into the blood
Show answer
Answer: A. Hydrogen nuclei (protons) in water and fat
Hydrogen nuclei behave like tiny magnets; in the strong field they absorb and re-emit radio waves, and soft tissues are full of hydrogen.
!Common mistakeChoosing calcium in bones is wrong: bone has little water, so it gives a weak MRI signal and looks dark, unlike in X-ray images.
7True or false
A person living in the mountains of northern Rwanda receives more cosmic radiation than a person living at sea level.
Show answer
Answer: True
At higher altitude there is less air above to absorb cosmic rays, so the cosmic-ray dose rate increases with height.
!Common mistakeLearners think background radiation is the same everywhere; it changes with altitude and with the rocks under the ground.
8Fill in the blank · ★ Challenge
If the linear attenuation coefficient of lead for a gamma beam is 1.2 cm⁻¹, its half-value thickness is ______ cm.
Show answer
Answer: 0.58
x½ = ln 2/μ = 0.693 ÷ 1.2 = 0.58 cm.
!Common mistakeWriting 1 ÷ 1.2 = 0.83 cm leaves out ln 2.
9True or false · ★ Challenge
A patient who absorbs 5.0 mGy of neutrons (wR = 10) receives an equivalent dose of 0.50 mSv.
Show answer
Answer: False
H = D × wR = 5.0 × 10 = 50 mSv, not 0.50 mSv.
!Common mistakeDividing by the weighting factor instead of multiplying makes the dangerous neutron dose look smaller than the absorbed dose.
10True or false
The tissue weighting factors of all the organs and tissues of the body add up to 1.
Show answer
Answer: True
The factors share out the whole-body risk among the organs, so if every organ gets the same equivalent dose H, the effective dose is also H.
!Common mistakeLearners sometimes think each factor can be up to 1; they are fractions of the whole-body risk, so they add to 1.
11Multiple choice · ★ Challenge
Why are bone marrow, the lining of the gut and an unborn baby especially sensitive to radiation?
- AThey contain more water, which becomes radioactive
- BTheir cells divide quickly, so damaged DNA is copied before repair
- CThey lie close to the skin, so they absorb more of the radiation dose
- DTheir cells have no system at all for repairing DNA
Show answer
Answer: B. Their cells divide quickly, so damaged DNA is copied before repair
Rapidly dividing cells have less time to repair DNA damage, and errors are copied into new cells.
!Common mistakeChoosing "water becomes radioactive" is wrong: X-rays and gamma rays do not make tissue radioactive; they damage molecules by ionisation.
12Fill in the blank · ★ Challenge
A tracer has an activity of 320 MBq when it is prepared at 06:00 and a half-life of 6.0 h. At 18:00 the same day its activity is ______ MBq.
Show answer
Answer: 80
12 h is two half-lives: 320 → 160 → 80 MBq.
!Common mistakeDividing 320 by 12 h or by 6 h (giving 27 or 53 MBq) treats the decay as linear; it halves every half-life.
13Fill in the blank
Damage to the DNA in sperm or egg cells, which may be passed on to future children, is called a ______ effect.
Show answer
Answer: genetic (hereditary)
Somatic effects harm the exposed person; genetic effects appear in their descendants.
!Common mistakeAnswering "somatic" is wrong: somatic effects affect only the body of the exposed person.
14Fill in the blank · ★ Challenge
A beta source gives a dose rate of 15 μGy per hour at a laboratory bench. A technician who works there for 40 minutes receives an absorbed dose of ______ μGy.
Show answer
Answer: 10
Dose = dose rate × time = 15 × (40 ÷ 60) = 10 μGy.
!Common mistakeMultiplying 15 by 40 (giving 600 μGy) uses minutes with a rate given per hour.
15Multiple choice
Brachytherapy is a form of radiotherapy in which:
- ASeveral external X-ray beams are aimed at the tumour
- BA drug is given that makes tumour cells glow on a screen
- CUltrasound is used to heat and destroy the tumour
- DSmall sealed sources are put inside or next to the tumour
Show answer
Answer: D. Small sealed sources are put inside or next to the tumour
Placing the source in or beside the tumour gives it a high dose while the dose falls off quickly in the healthy tissue around it.
!Common mistakeChoosing "several external beams" describes external beam therapy (teletherapy), not brachytherapy.
16Fill in the blank · ★ Challenge
A patient's treatment plan is 50 Gy given in fractions of 2.5 Gy, one fraction each weekday (5 per week). The treatment lasts ______ weeks.
Show answer
Answer: 4
Number of fractions = 50 ÷ 2.5 = 20; 20 ÷ 5 = 4 weeks.
!Common mistakeStopping at 20 gives the number of fractions, not the number of weeks.
17Fill in the blank · ★ Challenge
The annual dose limit for radiation workers is 20 mSv. A worker who receives 0.25 mSv per week for 48 working weeks has used ______ % of the limit.
Show answer
Answer: 60
Total = 0.25 × 48 = 12 mSv; 12 ÷ 20 × 100 = 60 %.
!Common mistakeForgetting to multiply by the number of weeks compares one week's dose with a whole year's limit.
18Multiple choice · ★ Challenge
Removing an electron from a typical molecule in air needs about 15 eV. Which of these photons can ionise air: visible light (2 eV), ultraviolet-C (5 eV), an X-ray photon (20 keV)?
- AAll three, since all are electromagnetic
- BThe ultraviolet-C and X-ray photons
- CNone of them, since photons have no charge
- DOnly the X-ray photon
Show answer
Answer: D. Only the X-ray photon
A single photon must carry at least 15 eV to ionise; only the 20 keV (20 000 eV) photon does.
!Common mistakeChoosing "ultraviolet-C and X-ray" assumes all ultraviolet is ionising; a 5 eV photon is below the 15 eV needed for air.
19Multiple choice
In a gamma camera, the lead collimator in front of the crystal:
- AChanges the gamma rays into flashes of visible light for the camera
- BSpeeds up the gamma rays so that they reach the crystal sooner
- CLets through only gamma rays moving along its holes, to sharpen the image
- DAbsorbs the tracer before it can reach the patient's organs
Show answer
Answer: C. Lets through only gamma rays moving along its holes, to sharpen the image
Only gamma rays travelling parallel to the holes reach the crystal, so each point of the crystal "sees" only the part of the body directly below it.
!Common mistakeChoosing "changes gamma rays into light" describes the scintillator crystal, not the collimator.
20Multiple choice · ★ Challenge
A nurse standing 0.50 m from a patient who has been given a gamma-emitting tracer receives 12 μSv per hour. If she moves to 1.5 m away, her dose rate becomes about:
- A4.0 μSv per hour
- B36 μSv per hour
- C0.44 μSv per hour
- D1.3 μSv per hour
Show answer
Answer: D. 1.3 μSv per hour
Inverse-square law: the distance is 3 times larger, so the dose rate is 1/3² = 1/9 of 12 = 1.3 μSv per hour.
!Common mistakeChoosing 4.0 μSv per hour divides by 3 instead of 3²; for a small source the dose rate falls with the SQUARE of the distance.
21Multiple choice
Which is a deterministic effect (a tissue reaction) of radiation?
- ABurning of the skin after a large dose
- BLeukaemia appearing years after a small dose
- CA gene mutation passed on to a child
- DThyroid cancer appearing 15 years later
Show answer
Answer: A. Burning of the skin after a large dose
Deterministic effects appear only above a threshold dose, and their severity grows with dose; skin burns are an example.
!Common mistakeChoosing leukaemia is tempting because it is serious, but cancers are stochastic: their probability, not their severity, depends on dose.
22Multiple choice · ★ Challenge
A gamma beam passes through a lead wall 4 half-value thicknesses thick and then through a concrete wall 2 half-value thicknesses thick. What percentage of the beam gets through both?
- A17 %
- B12.5 %
- C1.6 %
- D3.1 %
Show answer
Answer: C. 1.6 %
Six halvings in total: (1/2)⁶ = 1/64 = 0.016 = 1.6 %.
!Common mistakeChoosing 12.5 % multiplies the walls' effects as (1/4) × (1/2); each half-value thickness halves the beam, so 4 + 2 = 6 halvings.
23Multiple choice · ★ Challenge
The core of an endoscope fibre has refractive index 1.52 and its cladding 1.48. What is the critical angle at the core–cladding boundary?
- A41°
- B77°
- C13°
- D43°
Show answer
Answer: B. 77°
sin c = ncladding/ncore = 1.48 ÷ 1.52 = 0.974, so c = 77°.
!Common mistakeChoosing 41° uses sin c = 1/1.52, the critical angle for glass and air; the cladding, not air, surrounds the core.
24Multiple choice
In an ultrasound probe, the pulses are produced and their echoes detected by a:
- APiezoelectric crystal
- BLoudspeaker coil
- CHot tungsten filament
- DPhotoelectric metal plate
Show answer
Answer: A. Piezoelectric crystal
An alternating voltage makes a piezoelectric crystal vibrate at megahertz frequencies; returning echoes squeeze the crystal and produce a voltage.
!Common mistakeChoosing a loudspeaker coil is tempting because it makes sound, but it cannot vibrate at millions of times per second; a piezoelectric crystal can.
25Short answer · ★ Challenge
Radiotherapy is usually given as many small daily doses (fractions) over several weeks rather than as one large dose. Explain why.
Show answer
Model answer: Healthy cells repair sub-lethal radiation damage between fractions better than most cancer cells, so splitting the dose protects healthy tissue and reduces side effects. Between fractions, tumour cells that were resistant (for example short of oxygen or in a resistant stage of division) can become more sensitive, so more of them are killed overall.
!Common mistakeA common error is to think fractionation is only for the patient's comfort; its main reason is the different repair of healthy and cancer cells.
26Multiple choice
Light stays inside the glass fibres of an endoscope, even round bends, because of:
- ARefraction at the far end
- BDiffraction round the bends
- CTotal internal reflection
- DAbsorption by the fibre walls
Show answer
Answer: C. Total internal reflection
Light meets the core–cladding boundary at more than the critical angle, so it is completely reflected each time.
!Common mistakeChoosing refraction is tempting because the light does refract where it enters and leaves, but along the fibre it is kept in by total internal reflection.
27Multiple choice · ★ Challenge
Ultrasound of frequency 5.0 MHz travels at 1540 m/s in soft tissue. The smallest details it can show are about one wavelength across. How big is this?
- A3.1 mm
- B0.031 mm
- C7.7 mm
- D0.31 mm
Show answer
Answer: D. 0.31 mm
λ = v/f = 1540 ÷ 5.0 × 10⁶ = 3.1 × 10⁻⁴ m = 0.31 mm.
!Common mistakeChoosing 3.1 mm comes from a power-of-ten slip when changing metres to millimetres.
28Short answer · ★ Challenge
Explain why an MRI scanner is very noisy during a scan, and why patients must remove keys, coins and bank cards before entering the room.
Show answer
Model answer: Gradient coils inside the scanner have their currents switched on and off rapidly; in the very strong magnetic field these currents feel large forces, so the coils vibrate and make loud knocking sounds. The strong field pulls iron and steel objects such as keys and coins towards the magnet at high speed, where they can injure people, and it wipes the magnetic strip of bank cards.
!Common mistakeA common error is to think the noise is from the radio waves; radio waves are silent, the noise comes from vibrating coils.
29Multiple choice
The effective dose differs from the equivalent dose because it also takes into account:
- AThe type of radiation involved, using radiation weighting factors wR
- BThe length of time over which the dose was received
- CHow sensitive each exposed organ is, using tissue weighting factors
- DThe distance between the patient and the source
Show answer
Answer: C. How sensitive each exposed organ is, using tissue weighting factors
Equivalent dose already includes the radiation type; effective dose adds up the equivalent dose to each organ multiplied by its tissue weighting factor.
!Common mistakeChoosing "the type of radiation" is the commonest slip: that factor (wR) is already in the equivalent dose.
30Short answer · ★ Challenge
In a PET scan, detectors on opposite sides of the patient register two gamma photons at the same instant. Explain how this locates the tracer.
Show answer
Model answer: A positron from the tracer meets an electron almost at once and they annihilate. Their total momentum is about zero, so two 0.511 MeV photons fly off in opposite directions. When two opposite detectors fire together, the annihilation must have happened on the straight line joining them; many such lines cross at the place where the tracer is concentrated, and a computer builds a 3D image from them.
!Common mistakeA common error is to think each detector locates the tracer by itself; it is the coincidence of two detectors that gives the line.
31Multiple choice · ★ Challenge
A 5.0 MHz Doppler probe meets blood flowing at 0.30 m/s, at 60° to the beam. Using Δf = 2fv cos θ/c with c = 1540 m/s, what is the frequency shift?
- A1.9 kHz
- B0.49 kHz
- C1.7 kHz
- D0.97 kHz
Show answer
Answer: D. 0.97 kHz
Δf = 2 × 5.0 × 10⁶ × 0.30 × cos 60° ÷ 1540 = 3.0 × 10⁶ × 0.50 ÷ 1540 = 974 Hz ≈ 0.97 kHz.
!Common mistakeChoosing 1.9 kHz leaves out cos 60°; choosing 1.7 kHz uses sin 60° instead of cos 60°.
32Multiple choice
For most people in the world, the largest single source of natural background radiation is:
- ACosmic rays coming from the Sun and space
- BMedical X-rays and scans in hospitals
- CRadon gas seeping out of rocks and soil
- DLeaks from nuclear power stations
Show answer
Answer: C. Radon gas seeping out of rocks and soil
Radon-222 is an alpha emitter that collects in buildings; breathing it in gives about half of the average natural dose.
!Common mistakeChoosing nuclear power stations is a common belief, but their contribution to the public dose is a tiny fraction of a percent.
33Short answer · ★ Challenge
Alpha particles are the most ionising radiation, yet an alpha source outside the body is less dangerous than a gamma source of the same activity. Explain this, and say when alpha becomes the most dangerous.
Show answer
Model answer: Alpha particles travel only a few centimetres in air and are stopped by the dead outer layer of the skin, so from outside they do not reach living cells; gamma rays pass into the body and reach the organs. If an alpha emitter is breathed in (for example radon) or swallowed, all its energy is deposited in a very small volume of living tissue, so it then does the most damage.
!Common mistakeA common error is to rank danger only by ionising power; the danger also depends on whether the radiation can reach living cells.
34Multiple choice · ★ Challenge
A 15 kg child and a 75 kg adult each absorb 3.0 mJ of energy from the same X-ray examination. Which statement is correct?
- AThe child gets 0.20 mGy, five times the adult's 0.040 mGy
- BBoth get 3.0 mGy, since the energy absorbed is the same
- CThe adult gets 0.20 mGy, five times the child's 0.040 mGy
- DThe child gets 0.20 Gy, five times the adult's 0.040 Gy
Show answer
Answer: A. The child gets 0.20 mGy, five times the adult's 0.040 mGy
D = E/m: child 3.0 × 10⁻³ ÷ 15 = 2.0 × 10⁻⁴ Gy = 0.20 mGy; adult 3.0 × 10⁻³ ÷ 75 = 4.0 × 10⁻⁵ Gy = 0.040 mGy.
!Common mistakeChoosing "0.20 Gy" forgets that 3.0 mJ is 3.0 × 10⁻³ J; the same energy in a smaller mass gives a bigger dose, which is why children need lower settings.
35Multiple choice
Per centimetre of thickness, which material is the most effective shield against gamma rays?
- AConcrete
- BLead
- CWater
- DAluminium
Show answer
Answer: B. Lead
Gamma rays are absorbed most by dense materials of high atomic number; lead is both.
!Common mistakeChoosing concrete is tempting because thick concrete walls are used in hospitals, but concrete is chosen because it is cheap and strong; per centimetre it is much less effective than lead.
36Short answer · ★ Challenge
Explain why a Doppler probe must not be held at 90° to the blood vessel it is measuring.
Show answer
Model answer: At 90° the blood moves across the beam, not towards or away from the probe, so its velocity has no component along the beam. Since Δf = 2fv cos θ/c and cos 90° = 0, there is no frequency shift and the machine would read zero speed, even though the blood is flowing.
!Common mistakeA common error is to think 90° gives the strongest signal because the beam hits the vessel "straight on"; for Doppler it gives no shift at all.
37Multiple choice · ★ Challenge
Iodine-131 has a physical half-life of 8.0 days and a biological half-life in the thyroid of 24 days. What is its effective half-life? (1/Teff = 1/Tphys + 1/Tbio)
- A6.0 days
- B16.0 days
- C32 days
- D12 days
Show answer
Answer: A. 6.0 days
1/Teff = 1/8.0 + 1/24 = 3/24 + 1/24 = 4/24, so Teff = 6.0 days.
!Common mistakeChoosing 16 days averages the two half-lives; the effective half-life must be SHORTER than both, because decay and removal act together.
38Fill in the blank
During an MRI scan, hydrogen nuclei in the body absorb energy from pulses of radio-frequency waves and then re-emit it; the emitted signals are picked up by ______ placed around the patient.
Show answer
Answer: coils (receiver coils)
The returning radio signals induce small voltages in receiver coils; a computer turns these into an image.
!Common mistakeAnswering "a film" or "a gamma camera" mixes MRI with X-ray or nuclear imaging; MRI detects radio signals electrically.
39Multiple choice · ★ Challenge
During a scan, the lungs (tissue weighting factor 0.12) receive an equivalent dose of 5.0 mSv and the stomach (0.12) receives 2.0 mSv; no other organ is exposed. What is the effective dose?
- A7.0 mSv
- B0.84 mSv
- C0.42 mSv
- D3.5 mSv
Show answer
Answer: B. 0.84 mSv
E = Σ wT HT = 0.12 × 5.0 + 0.12 × 2.0 = 0.60 + 0.24 = 0.84 mSv.
!Common mistakeChoosing 7.0 mSv just adds the organ doses; each organ dose must first be multiplied by its tissue weighting factor.
40Multiple choice
Doppler ultrasound can measure how fast blood flows because the echo from moving red blood cells has:
- AA longer delay the faster the blood is moving
- BA changed frequency that depends on the blood speed
- CA larger amplitude the faster the blood is moving
- DThe same frequency but a different speed through the tissue
Show answer
Answer: B. A changed frequency that depends on the blood speed
Cells moving towards the probe return a higher frequency, cells moving away a lower one; the shift Δf is proportional to the speed.
!Common mistakeChoosing "a longer delay" confuses Doppler with echo ranging; the delay tells depth, while the frequency shift tells speed.
41Short answer · ★ Challenge
A learner says: "The gray and the sievert are both J/kg, so 1 Gy always equals 1 Sv." Explain when this is true and when it is false, with an example.
Show answer
Model answer: It is true only for radiation with a weighting factor of 1, such as X-rays, gamma rays and beta particles: 1 Gy gives 1 Sv. It is false for alpha particles (wR = 20) or neutrons (wR about 10): 1 Gy of alpha radiation gives an equivalent dose of 20 Sv, because the same energy does far more biological damage.
!Common mistakeThe error is to treat equal units as equal quantities; the sievert includes the biological effect of the radiation type.
42Short answer · ★ Challenge
Compare an ultrasound A-scan with a B-scan: how each is made, how it is displayed and one use of each.
Show answer
Model answer: A-scan: one beam in one fixed direction; the echoes are shown as spikes on a graph of amplitude against time; used to measure distances such as the length of the eyeball. B-scan: the beam is swept (or many transducers fire in turn); each echo is shown as a bright dot whose brightness depends on its strength, building a two-dimensional picture; used for pregnancy and abdominal scans.
!Common mistakeA common error is to think the A-scan is a picture; it is only a graph of echoes along one line.
43Short answer
A radiation worker's dosimeter badge has small windows covered by different filters (open, plastic, aluminium and lead). Explain why.
Show answer
Model answer: Different radiations are stopped by different filters: beta particles and low-energy X-rays are stopped by the plastic or aluminium, while high-energy gamma rays pass even through the lead. Comparing the reading (or film blackening) behind each window shows what type and energy of radiation the worker received, so the dose can be worked out correctly.
!Common mistakeA common error is to think the filters protect the worker; they only allow the badge to tell types of radiation apart.
44Short answer · ★ Challenge
Distinguish between stochastic and deterministic effects of radiation, and explain why there is said to be no completely safe dose for stochastic effects.
Show answer
Model answer: Stochastic effects (cancer, hereditary effects) are random: the chance of them happening increases with dose, but their severity does not. Deterministic effects (skin burns, cataracts, radiation sickness) only appear above a threshold dose and become more severe as the dose increases. For stochastic effects no threshold is assumed: even a small dose gives a small chance of damaging DNA, so doses are kept as low as reasonably achievable.
!Common mistakeLearners often think a small dose is "safe" because nothing is seen; stochastic effects may show up many years later, with a probability set by the dose.
45Short answer · ★ Challenge
The concrete walls of a radiotherapy bunker are 1.5 m thick. For the treatment beam the half-value thickness of concrete is about 12 cm. How many half-value thicknesses is this, and what fraction of the beam gets through?
Show answer
Model answer: Number of half-value thicknesses n = 150 ÷ 12 = 12.5. Fraction transmitted = (1/2)12.5 ≈ 1.7 × 10⁻⁴, less than 0.02 % of the beam.
!Common mistakeA common error is to divide 1.5 by 12 without changing to the same units; the thickness must be 150 cm to compare with 12 cm.
46Short answer
Explain why the effective half-life of a tracer in the body is always shorter than both its physical half-life and its biological half-life.
Show answer
Model answer: Two processes reduce the activity in the body at the same time: radioactive decay and removal by the body (for example in urine). Their rates add together, so the activity falls faster than it would by either process alone, and the time to halve is shorter than either half-life.
!Common mistakeA common error is to average the two half-lives; averaging gives a value between them, but the combined process is faster than both.
47Short answer · ★ Challenge
After treatment with iodine-131, a patient's thyroid (tissue weighting factor 0.04) receives an equivalent dose of 20 mSv and the rest of the body almost nothing. Calculate the effective dose and explain what this number means.
Show answer
Model answer: Effective dose = 0.04 × 20 = 0.80 mSv. It means the risk of cancer or genetic harm is about the same as if the WHOLE body had received an even dose of 0.80 mSv, so it can be compared with other examinations and with background radiation.
!Common mistakeA common error is to quote 20 mSv as the whole-body risk; only one small organ received it, so the effective dose is much lower.
48Short answer · ★ Challenge
An A-scan of an eye shows echoes from the front of the lens at 6.5 μs, the back of the lens at 12.0 μs and the retina at 31.0 μs after the pulse. Taking the speed of sound as 1550 m/s throughout, find the thickness of the lens and the distance from the probe to the retina.
Show answer
Model answer: Lens thickness = v × Δt ÷ 2 = 1550 × (12.0 − 6.5) × 10⁻⁶ ÷ 2 = 4.3 × 10⁻³ m = 4.3 mm. Probe to retina = 1550 × 31.0 × 10⁻⁶ ÷ 2 = 0.024 m = 24 mm.
!Common mistakeForgetting the ÷ 2 doubles both distances; each echo time is for the journey there and back.
49Short answer
An endoscope contains two bundles of fibres. Explain why the bundle that carries the image back must be "coherent", while the bundle that carries light into the body need not be.
Show answer
Model answer: In the image bundle each fibre carries one small dot of the picture, so the fibres must keep the same arrangement at both ends; otherwise the dots would be mixed up and the picture scrambled. The light-carrying bundle only has to deliver light to the inside of the body, so the order of its fibres does not matter.
!Common mistakeA common error is to think "coherent" here means coherent light as in a laser; for fibre bundles it means the fibres are kept in the same order.
50Short answer · ★ Challenge
At a cruising height of 11 km, airline passengers receive about 5 μSv per hour from cosmic rays. Estimate the dose for a 6-hour flight from Kigali, compare it with a chest X-ray (about 0.02 mSv) and explain why the dose rate is higher in the air than on the ground.
Show answer
Model answer: Dose = 5 × 6 = 30 μSv = 0.030 mSv, about one and a half chest X-rays. At 11 km there is much less atmosphere above the plane to absorb the cosmic rays and the particles they produce, so the dose rate is many times higher than at ground level.
!Common mistakeA common slip is mixing μSv and mSv: 30 μSv is 0.030 mSv, not 30 mSv.