Electromagnetic Spectrum

Every electromagnetic wave is transverse

Electromagnetic waves consist of changing electric and magnetic fields. Their oscillations are perpendicular to the direction of travel, so every region—from radio to gamma—is transverse.

They do not need a material medium. In a vacuum, every region travels at the same speed:

c = 3.0 × 10⁸ m s⁻¹

Frequency rises as wavelength falls

1. radio
2. microwave
3. infrared
4. visible
5. ultraviolet
6. X-ray
7. gamma

This strip shows order only. Equal box widths do not represent equal frequency or wavelength ranges, and the boxes do not mark numerical boundaries.

The relationship v = fλ explains the inverse trend: because all regions have the same vacuum speed, a larger frequency requires a smaller wavelength.

Use measurements without claiming more than they show

Worked classification: three records contain one radio, one infrared and one visible signal. Record M has wavelength 20 μm, N has 0.50 μm and P has 2 m. Convert to a common unit before comparing: M is 0.000020 m and N is 0.00000050 m. Wavelength decreases in the order P, M, N, so frequency increases in that order. The stated one-per-region condition identifies P as radio, M as infrared and N as visible. Without that condition, ordering three measurements alone would not identify their regions.

A measurement can also leave a range of possible wavelengths. Use its whole range, not just the midpoint. Numerical band boundaries in a question are supplied working conventions; you do not need to memorise them for this lesson. For example, NASA's remote-sensing introduction uses approximately 400–700 nm for visible light.

Use one fixed speed to connect frequency and wavelength

A wave's frequency counts cycles passing a point each second. In that second the pattern travels one wavelength per cycle, so its speed is frequency × wavelength. In a vacuum, use c = fλ with c = 3.0 × 10⁸ m s⁻¹. Convert the frequency to hertz before substituting.

Worked example: an illustrative radio signal has frequency 100 MHz = 1.00 × 10⁸ Hz. Its vacuum wavelength is λ = c/f = (3.0 × 10⁸)/(1.00 × 10⁸) = 3.0 m. If the frequency doubles, twice as many cycles fit into the same distance travelled in one second: the wavelength halves, while the speed stays fixed.

Applications of electromagnetic radiation

The prescribed examples above follow the linked syllabus. For the radiation mechanisms, see the IAEA description of gamma irradiation and its explanation of nuclear imaging and treatment.

Start with the job, not a memorised pair. Ask what the wave must do—travel through the atmosphere, be absorbed by water, reveal temperature, pass through soft tissue or cause ionisation—then choose the region whose interaction fits.

Choose from the conditions, then justify the choice

Worked comparison: a camera must locate a warm person in an unlit room without adding illumination. Choose a thermal infrared camera: it detects emitted infrared. An ordinary visible-light camera is less suitable because the scene supplies too little visible light. “Infrared is used in cameras” names a use but does not explain this decision.

For the transmission mechanism and the distinction between imaging and radiation risk, see the FDA explanation of X-ray imaging.

Absorbed non-ionising radiation can heat tissue

Radio waves, microwaves and infrared do not normally ionise atoms. At sufficiently high absorbed intensity, however, they transfer energy to tissue and raise its temperature. Microwaves can heat internal tissue; strong infrared can burn skin; intense visible light can damage the eye.

Ionising radiation can damage cells and DNA

X-rays and gamma rays can transfer enough energy in one interaction to remove electrons from atoms and molecules. This ionisation can damage cell structures and DNA, cause mutations that may lead to cancer, or kill cells at high dose.

Ultraviolet spans a range. At this level, describe its important photochemical skin and eye damage and increased cancer risk; do not claim that every ultraviolet frequency is ionising.

Connect the control to the exposure path

A useful explanation identifies the source, how radiation reaches a person, and the interaction that could cause harm. A control then interrupts that path or reduces exposure. The words “be careful” do not explain how it works.

Worked example: an enclosed UV water-treatment unit should irradiate water, not a nearby operator's eyes or skin. A UV-blocking enclosure blocks the exposure path; switching off the source before opening it removes the source during access. The useful damage to microorganisms does not imply that human tissue is unaffected.

The WHO summary of ultraviolet radiation describes skin and eye harm from excessive UV exposure and the risks of sunbeds. UV effects depend on wavelength and exposure; do not replace that distinction with “all UV has exactly the same effect”.

Common mistakes

Worked examples: select, justify and consider risk

Check your understanding

Guided practice

Independent self-check

1. State two properties shared by all electromagnetic waves.
2. Write all seven regions from longest wavelength to shortest wavelength.
3. Give one prescribed use each for radio, microwave, infrared and visible light.
4. Give one prescribed use each for ultraviolet, X-rays and gamma rays.
5. Explain one hazardous heating effect and one hazardous ionising effect on tissue.

Try this next

Close the answers, redraw the seven-region order from memory, then answer the use and hazard questions with different examples. Review only the missed relationship and retry before moving to Electric charge and current.

Practise this topic

Start with the topic check. Use the feedback to return to the right explanation, then try a fresh check later.