OxfordAQA International GCSE Physics: The electromagnetic spectrum: uses, hazards, infrared and X-rays (9203) – Revision Notes

These revision notes condense section 3.3.2, The electromagnetic spectrum (statements a to m), of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. The qualification is untiered. Statements e and f, and the emission detail in d, are for the full Physics award. For full explanations read the study guide first, then try the practice questions.

Course pages: OxfordAQA IGCSE Physics hub, printable checklist and the free 10-minute diagnostics.

Key facts at a glance

Statement	Fact to recall
3.3.2 a	EM waves are transverse; a source passes energy to an absorber by them
3.3.2 b	Continuous spectrum; same speed in a vacuum; wavelengths from 10⁻¹⁵ m to 10⁴ m and beyond
3.3.2 c	Visible light is what the eye detects; different wavelengths look like different colours
3.3.2 d	All objects emit and absorb infrared; hotter means more radiated in a given time
3.3.2 e, f	Hotter means more infrared and higher frequencies; black-body radiation is the range emitted at a given temperature
3.3.2 g, h	Communication and application lists
3.3.2 i	Low energy: heating. High energy: ionisation
3.3.2 j to m	X-ray properties, diagnosis, treatment, precautions

The order of the spectrum

Group	Frequency and energy	Wavelength
Radio waves	lowest	longest
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays	highest	shortest

Within visible light, red has the longest wavelength and violet the shortest.

Must-know distinction. Speed is the same for all groups in a vacuum. Frequency, wavelength and energy differ. Higher frequency means shorter wavelength and more energy.

Wave equation method

wave speed = frequency × wavelength, with c = 3.00 × 10⁸ m/s

1. Write the known quantity and its unit.
2. Convert to metres or hertz (nm × 10⁻⁹, µm × 10⁻⁶, kHz × 10³, MHz × 10⁶, GHz × 10⁹).
3. Rearrange: λ = c / f or f = c / λ.
4. Substitute, calculate, and give a sensible number of significant figures with the unit.

Worked reminder 1

A radio station transmits at 100 MHz. Wavelength?

1. f = 100 × 10⁶ = 1.00 × 10⁸ Hz.
2. λ = (3.00 × 10⁸) / (1.00 × 10⁸) = 3.0 m.

Worked reminder 2

An infrared signal has wavelength 2.5 µm. Frequency?

1. λ = 2.5 × 10⁻⁶ m.
2. f = (3.00 × 10⁸) / (2.5 × 10⁻⁶) = 1.2 × 10¹⁴ Hz.

Worked reminder 3

A wave has frequency 6.0 × 10¹⁶ Hz. Wavelength, and which group?

1. λ = (3.00 × 10⁸) / (6.0 × 10¹⁶) = 5.0 × 10⁻⁹ m (5.0 nm).
2. The short wavelength and very high frequency place it among the ultraviolet and X-ray groups. The boundaries overlap, so say that it is at the high-energy end.

Infrared radiation

• All objects emit and absorb infrared radiation.
• Physics award: objects emit infrared because of the motion of their particles. The amount and frequency depend on temperature and surface.
• Hotter object: more infrared radiated in a given time.

Surface	Absorber	Emitter	Reflector
Dark, matt	good	good	poor
Light, shiny	poor	poor	good

Experiment pattern. Same mass and starting temperature of water, same size and material of can, same distance from the heater. Change only the surface. Compare temperature rises when heating, or temperature falls when cooling.

Reasoning pattern. “The matt black can’s temperature rose more, so it absorbed more infrared.” Link the observation to the property.

Black-body radiation (Physics only)

• As an object heats up it radiates more infrared and radiates at higher frequencies.
• Black-body radiation is the range of electromagnetic radiation emitted by an object at a particular temperature.
• A warm object gives out mainly infrared. A much hotter object also gives out visible light, first red, then brighter and whiter as the range reaches higher frequencies.

Matching a property to a use

• Gamma rays: ionising, so they kill bacteria on food and on instruments.
• Infrared: emitted by warm bodies, so night vision devices can detect people in the dark.
• Visible light: carried along fibre optic cables for communication.
• X-rays: absorbed by bone and transmitted by soft tissue, so they give medical images.
• Ultraviolet: used in security marking.

Uses

Wave	Uses to learn
Radio waves	Television and radio, Bluetooth
Microwaves	Mobile phones, satellite television
Infrared	TV remote controls, night vision devices, heating
Visible light	Photography, fibre optic communications
Ultraviolet	Security marking
X-rays	Medical imaging
Gamma rays	Sterilising surgical instruments, killing harmful bacteria in food

Communication: radio waves, microwaves, infrared and visible light.

Hazards and protection

Wave	Hazard	Protection
Microwaves	Heating of body tissue	Sealed, shielded ovens and transmitters kept away from people
Infrared	Skin burns	Keep away from hot objects, protective clothing
Ultraviolet	Skin cancer and blindness	Sunscreen, cover up, UV-blocking eyewear
X-rays	High doses kill cells	Lead shielding, short exposure, distance
Gamma rays	Genetic mutations	Thick lead or concrete, short time, distance

Rule. Low-energy waves: heating. High-energy waves: ionisation, which is the removal of an electron from an atom or molecule.

X-rays

• Very short wavelength, high energy, cause ionisation.
• Affect photographic film like light.
• Absorbed strongly by metal and bone; transmitted by healthy tissue.
• Diagnosis: bone fractures, dental problems, CT scanning.
• Treatment: killing cancer cells.

Method in steps for explaining an X-ray image.

1. The X-rays pass through the body to the film or detector.
2. Soft tissue transmits most of them, so that part of the film is exposed.
3. Bone absorbs them strongly, so less reaches the film behind it.
4. The difference shows the bone, and a break appears as a gap or line.

Precautions with ionising radiation

Monitor and minimise exposure for people who work with it:

• less time near the source;
• more distance from the source;
• shielding such as lead aprons and screens;
• a monitoring badge worn to record exposure.

Worked reminder 4: reading experiment data

Two identical beakers of water start at 90 °C. After 8 minutes the one wrapped in matt black paper is at 66 °C and the one wrapped in shiny foil is at 78 °C.

1. Fall for the black beaker = 90 − 66 = 24 °C. Fall for the foil beaker = 90 − 78 = 12 °C.
2. The black beaker lost energy twice as fast (24 / 12 = 2.0).
3. Conclusion: dark, matt surfaces are better emitters of infrared than light, shiny ones.

Command words in this topic

Command word	Aim your answer at
State	a single fact: a wave, a hazard or a use
Describe	what happens, step by step, naming the waves or surfaces
Explain	the reason, joined to the property with “because”
Calculate	equation, converted units, answer with unit
Suggest	applying a known property to a new situation

Quick self-test

1. State two features shared by all electromagnetic waves.
2. Put these in order of increasing frequency: ultraviolet, radio waves, infrared, gamma rays, microwaves.
3. A radio wave has frequency 100 MHz. Calculate its wavelength.
4. Calculate the frequency of infrared of wavelength 2.5 µm.
5. Which type of surface is the best emitter of infrared?
6. State what happens to the radiation emitted by an object as it gets hotter.
7. State the hazard of ultraviolet.
8. State one use of gamma rays.
9. Explain why bone shows up on an X-ray image.
10. State two precautions for a radiographer.

Answers

1. Transverse; one common speed in a vacuum. (Also acceptable: they transfer energy from a source to an absorber.)
2. Radio waves, microwaves, infrared, ultraviolet, gamma rays.
3. λ = (3.00 × 10⁸) / (1.00 × 10⁸) = 3.0 m.
4. f = (3.00 × 10⁸) / (2.5 × 10⁻⁶) = 1.2 × 10¹⁴ Hz.
5. A dark, matt surface.
6. It radiates more infrared in a given time, and also radiates at higher frequencies (Physics award).
7. Skin cancer and blindness.
8. Sterilising surgical instruments, or killing harmful bacteria in food.
9. Bone absorbs X-rays strongly while soft tissue transmits them, so less radiation reaches the film behind the bone.
10. Any two from: stand behind a lead screen or leave the room, wear a lead apron, limit the time of exposure, increase the distance, wear a monitoring badge.

Where marks are usually lost

• Calling the groups by the wrong order, especially microwaves and infrared.
• Not converting units before using the wave equation.
• Saying that only dark objects emit infrared, instead of “all objects emit and absorb it”.
• Describing a result without linking it to the surface property, for example “the black can cooled faster” with no mention of emission.
• Not controlling or mentioning variables in an experiment description.
• Writing “radiation is harmful” with no named wave and no named effect.
• Saying that X-rays are “blocked” by flesh, or that they pass through bone.
• Giving “wear protective clothing” without saying what it protects against.

Official syllabus

OxfordAQA International GCSE Physics (9203), for exams May/June 2018 onwards, published by OxfordAQA, section 3.3.2 The electromagnetic spectrum.