4.7.1Electrons, Waves and Photons
The Electromagnetic Spectrum — OCR A Level Physics A H556 Revision Notes & Topical Questions
5 min read 5 parts Lesson 47 of 62 Unit quiz
Part 1 of 5Common Properties
Radio waves, light and gamma rays are all the same kind of wave, differing only in wavelength and frequency.
Transverse
- All electromagnetic waves are transverse, with electric and magnetic fields oscillating at right angles to each other and to the direction of travel.
Vacuum
- They can travel through a vacuum, so no medium is needed, which is how sunlight reaches the Earth.
Speed
- All travel at the same speed in a vacuum, about 3.00 × 10⁸ m s⁻¹, and nearly the same speed in air.
Behaviour
- They can reflect, refract, diffract, interfere and be polarised, as all transverse waves can.
Part 2 of 5Order of the Spectrum
1
Radio waves have the longest wavelengths, roughly above 0.1 m, and the lowest frequencies.
2
Microwaves come next, with wavelengths from about 1 mm up to 0.1 m.
3
Infrared radiation follows, from about 700 nm up to 1 mm, with visible light from about 400 nm to 700 nm after it.
4
Ultraviolet lies below 400 nm, and X-rays and gamma rays come after it with wavelengths below about 10 nm.
5
Moving from radio to gamma, the wavelength decreases while the frequency and photon energy increase.
Part 3 of 5Uses
Uses
Radio and microwaves
- Broadcasting and communication use radio waves, and microwaves are used for satellite links and cooking.
Infrared
- Infrared is used in remote controls, thermal imaging and short-range data transfer.
Visible light
- Visible light is detected by the eye and is used in fibre-optic communication and photography.
Ionising radiation
- Ultraviolet, X-rays and gamma rays can ionise atoms, so exposure to them must be limited.
| Region | Approximate wavelength | Example use |
|---|---|---|
| Radio | > 0.1 m | Broadcasting |
| Microwave | 1 mm to 0.1 m | Satellite links |
| Infrared | 700 nm to 1 mm | Thermal imaging |
| Visible | 400 nm to 700 nm | Vision |
| Ultraviolet | < 400 nm | Sterilisation |
| X-ray and gamma | < 10 nm | Medical imaging |
Part 4 of 5Worked Example
Question Calculate the frequency of yellow light of wavelength 590 nm in a vacuum, using c = 3.00 × 10⁸ m s⁻¹.
- Convert590 nm = 5.90 × 10⁻⁷ m.
- Rearrange v = fλf = c/λ.
- f = 3.00 × 10⁸ / 5.90 × 10⁻⁷ = 5.08 × 10¹⁴ Hz.
Answer Frequency = 5.1 × 10¹⁴ Hz.
Part 5 of 5Exam Checklist & Tips
4.7.1 – Key Points for the Exam
- Electromagnetic spectrum The family of waves ranging from radio waves to gamma rays.
- Transverse wave A wave in which oscillations are perpendicular to the direction of travel.
- Ionising radiation Radiation with enough energy per photon to remove electrons from atoms.
- Speed of light About 3.00 × 10⁸ m s⁻¹ in a vacuum.
Common Mistakes
- Saying electromagnetic waves need a medium to travel through.
- Putting the regions in the wrong order or mixing up microwaves and infrared.
- Believing different electromagnetic waves travel at different speeds in a vacuum.
- Forgetting that shorter wavelength means higher frequency.
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