Uses and Dangers of X-Rays and Lasers – X-Rays and Laser Radiations (Senior 6 Physics) | Ganzaa

X-Rays and Laser Radiations

Lessons

Senior 6 · Physics

X-Rays and Laser Radiations

4 lessons

Lesson 4 of 4

Uses and Dangers of X-Rays and Lasers

25 min

Two beams, two jobs

At an airport, your bag rolls through an X-ray scanner, while at the checkout a red laser reads the barcode on a bottle of water. Both are electromagnetic radiation made by electrons in atoms, but they do completely different jobs, and they are dangerous in completely different ways. X-rays are used because they pass THROUGH things; laser light because it can be aimed and focused with great precision. This lesson surveys the uses of each, explains the dangers and the safety rules that follow from the physics of Lessons 1 to 3, and ends by comparing the two.

Using X-rays

Every use depends on one property from Lesson 1: X-rays penetrate matter, and are absorbed more strongly by materials of higher atomic number and density.

Figure 4.1: Forming a shadow image (radiograph).

Field	Use	Physics
medicine	radiographs of bones, teeth and chests; CT scans (Unit 8)	bone (calcium) absorbs more than soft tissue; contrast agents containing iodine or barium make blood vessels or the gut visible
medicine	radiotherapy for cancer	high-energy X-rays from a linear accelerator damage tumour cells, aimed from several directions to spare healthy tissue
security	baggage and cargo scanners	metals and dense materials show as dark shadows
industry	checking welds, pipes and castings for cracks	a crack absorbs less, so it shows up on the image
science	X-ray crystallography	diffraction ( $2d\sin\theta = n\lambda$ ) reveals the arrangement of atoms in crystals, proteins and medicines
astronomy	X-ray telescopes in space	the atmosphere absorbs X-rays, so the telescopes must orbit above it

The dangers of X-rays

X-rays are ionising: each photon carries enough energy to break chemical bonds, including those in DNA. The World Health Organization (fact sheet "Ionizing radiation and health effects", 2023) describes two kinds of harm:

above a threshold dose, acute effects such as skin redness, hair loss, radiation burns and radiation sickness; the early X-ray workers, who did not know the danger, suffered burns to their hands;
at lower doses, a risk of long-term effects such as cataracts and cancer, years later. The risk rises with the dose and is higher for children, adolescents and unborn babies.

A single medical radiograph gives a very small dose, and its benefit usually far outweighs the risk. The international Basic Safety Standards, which the WHO co-sponsors, set the rules: every exposure must be justified (it must do more good than harm) and optimised (the dose kept as low as reasonably achievable while still giving the information needed). In practice:

the beam is limited by a collimator to the part being examined, and lead aprons or collars shield other organs;
the radiographer stands behind a lead-lined screen or outside the room;
women who may be pregnant are asked before an examination;
staff wear dosimeters to monitor their exposure (Unit 8);
X-ray rooms have lead-lined walls and warning lights.

Worked example 1: distance protects the radiographer

The X-rays scattered from a patient spread out from a small region, roughly like a point source. A radiographer who stood 0.50 m from the patient moves to 2.0 m away during the exposure. By what factor is her exposure reduced, and what assumption is made?

Solution

For a point source, intensity $\propto \frac{1}{r^2}$ (Unit 5). The factor is $\left(\frac{2.0}{0.50}\right)^2 = 4.0^2 = 16$ : her exposure falls to one sixteenth.

Assumption: the scattered radiation behaves like a point source and is not absorbed or reflected on the way. In practice she also stands behind a lead screen, which reduces it much further.

Using lasers

Lasers are used because their light is monochromatic, coherent, directional and intense (Lesson 3).

Field	Use	Property used
medicine	reshaping the cornea to correct eyesight (with ultraviolet lasers, 193 nm); sealing the retina; removing tumours, tattoos and birthmarks; dentistry	intensity focused precisely; the wavelength is chosen to be absorbed by the target tissue
communications	sending internet and telephone signals along optical fibres as pulses of infrared light (about 1310 nm and 1550 nm)	a narrow, monochromatic beam that enters a thin fibre and spreads little in time
industry	cutting, welding and engraving metal, cloth and wood	high intensity at a tiny focal spot
everyday technology	barcode scanners, laser printers, optical discs (CD 780 nm, DVD 650 nm, Blu-ray 405 nm)	a beam focused to a spot about the size of its wavelength: a shorter wavelength reads smaller pits, so more data fits on a disc
measurement	surveying and range-finding, LIDAR, measuring the distance to the Moon	directional beam; timing a pulse's round trip
science	holography, spectroscopy, detecting gravitational waves (LIGO, 2015)	coherence and a precisely known wavelength

An industrial laser cutting sheet metal under computer control. The bright sparks at the lower left come from metal melted and blown away at the focus of the beam. Photo: Mercurybds, CC BY 3.0, via Wikimedia Commons

Worked example 2: timing a laser pulse to the Moon

Laser pulses fired from Earth bounce off reflectors left on the Moon by the Apollo astronauts. A pulse returns 2.56 s after it was sent. (a) Find the distance to the Moon. (b) The return is timed to $1 \times 10^{-11}$ s. What uncertainty in distance does this give?

Solution

(a) The light travels there and back: $d = \frac{ct}{2} = \frac{(3.00 \times 10^{8})(2.56)}{2} = 3.84 \times 10^{8}\,\mathrm{m}$ , about 384 000 km.

(b) $\Delta d = \frac{c\,\Delta t}{2} = \frac{(3.00 \times 10^{8})(1 \times 10^{-11})}{2} = 1.5 \times 10^{-3}\,\mathrm{m}$ , about 1.5 mm. Only a laser's narrow beam can deliver enough photons to the reflectors, and back, for such a measurement.

Worked example 3: why a laser can cut steel

A 2.0 kW carbon dioxide laser is focused to a spot 0.20 mm in diameter. Find the intensity at the spot and compare it with sunlight (about $1.0 \times 10^{3}\,\mathrm{W\,m^{-2}}$ ).

Solution

$I = \frac{P}{\pi r^2} = \frac{2.0 \times 10^{3}}{\pi(0.10 \times 10^{-3})^2} = 6.4 \times 10^{10}\,\mathrm{W\,m^{-2}}$ , about 60 million times the intensity of sunlight. Delivered onto a tiny area, this heats the metal past its melting point in milliseconds, while the material a millimetre away stays fairly cool.

The dangers of lasers

Ordinary laser light is non-ionising: a visible photon carries only about 2 to 3 eV. Its danger is the concentration of energy. The greatest risk is to the eye.

Figure 4.2: The eye's own lens makes a laser beam dangerous.

Visible and near-infrared light is focused onto the retina, causing burns and permanent blind spots. Near-infrared beams are invisible, so the blink reflex gives no protection.
Ultraviolet and far-infrared beams are absorbed at the front of the eye and can damage the cornea and lens.
High-power beams also burn skin and can start fires.

Lasers are sold in safety classes (international standard IEC 60825-1). The main classes are shown, simplified, for a continuous visible beam; the standard also has classes 1M and 2M for beams that are hazardous only through magnifying optics:

Class	Typical power (visible beam)	Hazard
1	very low, or fully enclosed (as in a laser printer)	safe in normal use
2	under 1 mW	safe for a brief accidental glance, because the eye's natural aversion response (blinking, turning away) limits it; never stare into the beam
3R	up to 5 mW	low risk, but do not look into the beam
3B	up to 500 mW	the direct beam or a mirror reflection can injure the eye
4	above 500 mW	eye and skin injury, even from scattered light; fire risk

Safety rules: never look into a beam or point it at anyone; keep beams below eye level; wear goggles made for the laser's specific wavelength when using Class 3B and 4 lasers; and never aim any laser at an aircraft or a vehicle, because even a small pointer can dazzle a pilot or driver at a critical moment.

Comparing X-rays and lasers

Feature	X-rays (from a tube)	Laser light
nature	electromagnetic waves	electromagnetic waves
typical wavelength	0.01 nm to 10 nm	about 200 nm (ultraviolet) to 10 μm (infrared)
photon energy	about 100 eV to 100 keV: ionising	about 0.1 eV to 6 eV: non-ionising
how produced	fast electrons decelerated by nuclei, or falling into inner-shell vacancies	stimulated emission between outer energy levels, with a population inversion
spectrum	continuous, plus characteristic lines	one very narrow line
coherence	incoherent	coherent
beam	spreads out; shaped by lead collimators	narrow, parallel, easily focused
main hazard	ionisation of cells: cancer risk, burns at high doses	heating: eye and skin burns
protection	justification, short time, distance, lead shielding	never look into the beam, safety class, goggles for the wavelength

The two worlds meet in the X-ray free-electron laser, in which electrons from a linear accelerator are made to emit coherent X-ray pulses so short and bright that they can photograph molecules in motion.

Warning

Common mistakes. (1) Saying laser light is dangerous because it is "radiation" like X-rays: visible laser light is non-ionising; its danger is concentrated heating. (2) Thinking X-rays make a person or object radioactive: they do not; the exposure stops when the beam is switched off. (3) Thinking a reflection of a powerful laser is safe: reflected, and for Class 4 even scattered, light can injure. (4) Saying a lead apron makes an X-ray examination risk-free: it shields other organs, but the examination must still be justified.

Tip

To decide which tool a job needs, ask: "Through, or onto?" To see THROUGH something, use X-rays (they penetrate). To act ONTO a precise point, or to carry a signal far, use a laser (it is directional and focusable).

Why Astronauts left a reflector on the Moon! (Smarter Every Day, with MinutePhysics). Watch how laser pulses are fired at the Apollo reflectors and timed on their return to measure the distance to the Moon.

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Practice

Explain why bones appear white on a radiograph while the lungs appear dark.
Why are X-ray telescopes placed on satellites rather than on mountains?
State the two principles of the Basic Safety Standards for medical exposures, and give two practical ways a hospital keeps doses low.
A technician standing 1.5 m from a source of scattered X-rays moves to 3.0 m. By what factor does her exposure change?
A laser pulse sent to a reflector on a hill returns after $4.0 \times 10^{-6}$ s. How far away is the reflector?
A Blu-ray player uses a 405 nm laser and a CD player a 780 nm laser. Suggest why a Blu-ray disc can hold much more data.
Explain why a 5 mW laser pointer can damage the retina, while a 60 W lamp seen from across the room does not.
Gatete says: "Lasers and X-rays are both dangerous for the same reason: they are both radiation." Evaluate his statement.

Show answersHide answers

Bone contains calcium (higher atomic number) and is denser, so it absorbs more X-rays and fewer reach the detector behind it (white). The lungs are mostly air, so X-rays pass freely and the detector is strongly exposed (dark).
The atmosphere absorbs X-rays from space, so they never reach the ground, even on a mountain.
Justification (benefit must outweigh risk) and optimisation (dose as low as reasonably achievable). Examples: collimating the beam, lead aprons, staff behind lead screens, checking for pregnancy, monitoring staff doses.
Distance doubled: exposure falls by $2^{2} = 4$ , to a quarter.
$d = \frac{ct}{2} = \frac{(3.00 \times 10^{8})(4.0 \times 10^{-6})}{2} = 600\,\mathrm{m}$ .
A laser can be focused to a spot about the size of its wavelength. The shorter wavelength (about half) gives a smaller spot, so smaller pits can be read and packed more closely: roughly four times as many per unit area.
The laser's 5 mW is in a narrow parallel beam that the eye focuses onto a spot about 10 μm across, giving an enormous intensity. The lamp's light spreads in all directions, only a tiny fraction enters the eye, and it forms an extended image, not a point.
Only partly true. Both are electromagnetic radiation, but X-rays are dangerous because they are IONISING (they damage DNA, raising cancer risk). Visible and infrared laser light is non-ionising; its danger is the HEATING caused by a concentrated beam, mostly to the eyes and skin. The protection measures differ accordingly.

Key points

X-ray uses rely on penetration and absorption that grows with Z: radiography, radiotherapy, security, industry, crystallography.
X-rays are ionising: acute effects at high doses, cancer and cataract risk at lower doses (higher for children). Justify, optimise, collimate, shield, keep distance.
Laser uses rely on direction, focus, coherence and a single wavelength: surgery, fibre communication, cutting, discs, ranging.
Laser hazard is heating, mostly of the eye; know the class, never look into or point the beam.

1Two beams, two jobs

2Using X-rays

3The dangers of X-rays

4Using lasers

5The dangers of lasers

6Comparing X-rays and lasers

Two beams, two jobs

Using X-rays

The dangers of X-rays

Using lasers

The dangers of lasers

Comparing X-rays and lasers