OCR GCSE Physics A: Electromagnetic waves and the spectrum (J249) – Revision Notes

These notes condense statements P5.2a to P5.2f of topic P5.2, The electromagnetic spectrum, from the OCR GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026). All six statements are assessed in both Foundation and Higher Tier papers. For full explanations and longer worked examples, read the study guide. When you are ready to test yourself, use the practice questions. Course pages: OCR GCSE Physics hub and the printable checklist. The free 10-minute diagnostics show which topics to revise first.

Key ideas at a glance

Statement	Remember
P5.2a	EM waves are transverse. They cross empty space. All have the same speed in a vacuum.
P5.2b	They carry energy from a source to an absorber.
P5.2c	v = f × λ. With v fixed, f and λ are inversely proportional.
P5.2d	Radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays: long to short wavelength, low to high frequency.
P5.2e	The eye detects only a narrow band, the visible range.
P5.2f	Light is an EM wave.

Definitions

• Transverse wave: the oscillations are at right angles to the direction of travel of the wave.
• Wavelength (λ): the distance from one point on a wave to the same point on the next wave, for example crest to crest. Unit: metre (m).
• Frequency (f): the number of waves passing a point each second. Unit: hertz (Hz).
• Source: where the wave starts and from where energy leaves.
• Absorber: the material that takes in the energy of the wave.
• Electromagnetic spectrum: the continuous family of EM waves, grouped by wavelength and frequency.

The spectrum in order

Longest wavelength to shortest:

1. Radio
2. Microwave
3. Infrared
4. Visible (red, orange, yellow, green, blue, violet)
5. Ultraviolet
6. X-rays
7. Gamma rays

Same list read the other way gives low frequency to high frequency. The two directions always match: long wavelength means low frequency.

Rounded size guide (boundaries differ between sources):

Group	Wavelength scale
Radio	metres to kilometres (down to about 0.1 m)
Microwave	millimetres to centimetres
Infrared	about 1 mm down to 700 nm
Visible	about 700 nm (red) to 400 nm (violet)
Ultraviolet	400 nm down to about 10 nm
X-rays	about 10 nm to 0.01 nm
Gamma rays	shorter than about 0.01 nm

Shared features: transverse, same speed in a vacuum, transfer energy. Different features: wavelength, frequency, how they are absorbed, how they are produced and detected.

Speed, time and distance

All EM waves: 3.0 × 10⁸ m/s in a vacuum. If the question gives a value, use that value.

Method in steps:

1. Write the equation: time = distance ÷ speed.
2. Put the distance in metres.
3. Divide.
4. Give the unit and a sensible number of significant figures.

Reminder. A pulse covers 6.0 × 10⁷ m. Time = 6.0 × 10⁷ ÷ 3.0 × 10⁸ = 0.20 s.

Frequency and wavelength

v = f × λ        f = v / λ        λ = v / f

Method in steps:

1. Convert to hertz (Hz) and metres (m).
2. Write the form of the equation you need.
3. Substitute with v = 3.0 × 10⁸ m/s (or the value given).
4. Calculate and give the unit.
5. Ask: does the size fit the group?

Prefix	Multiply by
kilo (k)	10³
mega (M)	10⁶
giga (G)	10⁹
nano (n)	10⁻⁹
micro (μ)	10⁻⁶

Reminder 1. A microwave signal has frequency 10 GHz. f = 1.0 × 10¹⁰ Hz, so λ = 3.0 × 10⁸ ÷ 1.0 × 10¹⁰ = 0.030 m.

Reminder 2. Green light has wavelength 560 nm = 5.6 × 10⁻⁷ m, so f = 3.0 × 10⁸ ÷ 5.6 × 10⁻⁷ = 5.4 × 10¹⁴ Hz.

Inverse proportion shortcut. If one wave has half the wavelength of another in a vacuum, it has double the frequency. If its wavelength is ten times smaller, its frequency is ten times greater.

Microwave practical (spacing of melted spots)

Melted spots in food are half a wavelength apart. So wavelength = 2 × spacing. Measure across several gaps and divide by the number of gaps.

Energy transfer

• Say: the wave transfers energy from the source to the absorber.
• Name the source, the absorber and the effect on the absorber.
• Examples: microwaves from an oven emitter heat the water in food; infrared from a heater warms skin; sunlight warms a dark surface; radio waves from a transmitter reach a receiving aerial.
• A wave transfers energy. It does not transfer matter from source to absorber.

The eye and light

• The eye detects only visible light, about 400 nm to 700 nm.
• We cannot see infrared or ultraviolet because the eye is not a detector for those waves, not because they do not exist.
• Red has the longest wavelength and lowest frequency in the visible range. Violet has the shortest wavelength and highest frequency.
• Light is an EM wave: transverse, crosses a vacuum, 3.0 × 10⁸ m/s there, transfers energy.

Worked reminder: a two-step question

Question. A wave of frequency 6.0 × 10¹⁴ Hz travels through a vacuum. Calculate its wavelength in nanometres and say which group it belongs to.

λ = v / f = 3.0 × 10⁸ / 6.0 × 10¹⁴
  = 5.0 × 10⁻⁷ m
  = 500 × 10⁻⁹ m = 500 nm

500 nm lies inside the visible range, so this is visible light, in the green region. The second step is a size check against the table above. Always do it, because a wrong power of ten shows up at once when the group does not fit.

Writing a good explanation

For a “describe” or “explain” question on this topic, build the answer from short, linked statements:

• Name the group and say where it sits in the spectrum (for example, shorter wavelength than visible light, so higher frequency).
• State what is shared: transverse, same speed in a vacuum.
• State what differs: wavelength and frequency, and so how the wave is absorbed.
• If energy is involved, finish with source, wave, absorber and effect.

Must-know distinctions

Compare	Key difference
EM wave and sound	EM is transverse and crosses a vacuum. Sound is longitudinal and needs a medium.
Wavelength and frequency	Wavelength is a distance. Frequency is a rate in Hz.
Speed and frequency	All EM waves share one speed in a vacuum. Frequency varies across the spectrum.
Source and absorber	The source gives out energy. The absorber takes it in.
Red and violet	Red: longer wavelength, lower frequency. Violet: shorter wavelength, higher frequency.

Quick self-test

1. State the two properties shared by all EM waves that you have studied here, other than carrying energy.
2. List the seven groups from longest to shortest wavelength.
3. Which has the higher frequency, ultraviolet or infrared?
4. A long-wave radio signal has wavelength 1500 m. Calculate its frequency.
5. Infrared radiation has wavelength 10 μm. Calculate its frequency.
6. A gamma ray has frequency 2.0 × 10²⁰ Hz. Calculate its wavelength.
7. Wave P has frequency 4 times that of wave Q (both EM, in a vacuum). Compare the wavelengths.
8. Why can you not see the infrared from a heater element that is warm but not glowing?
9. State what is meant by saying that EM waves transfer energy from a source to an absorber.
10. Say whether light travels faster than X-rays in a vacuum.

Answers

1. They are transverse, and they travel at the same speed in a vacuum.
2. Radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
3. Ultraviolet.
4. f = 3.0 × 10⁸ ÷ 1500 = 2.0 × 10⁵ Hz (200 kHz).
5. λ = 10 × 10⁻⁶ = 1.0 × 10⁻⁵ m. f = 3.0 × 10⁸ ÷ 1.0 × 10⁻⁵ = 3.0 × 10¹³ Hz.
6. λ = 3.0 × 10⁸ ÷ 2.0 × 10²⁰ = 1.5 × 10⁻¹² m.
7. Wave P has a wavelength 4 times smaller (one quarter of the wavelength of Q).
8. Infrared has a wavelength outside the narrow range the eye detects, so the eye does not respond to it.
9. The wave starts at the source, crosses the gap and the absorber takes in the energy, for example by warming up.
10. No. Light and X-rays have the same speed in a vacuum.

Where marks are usually lost

• Writing sound as an example of an EM wave, or saying an EM wave needs air.
• Giving the order of the groups from short wavelength to long when the question asks long to short, or the reverse.
• Mixing up frequency and wavelength when naming which is higher for ultraviolet.
• Leaving MHz, GHz, nm or μm unconverted in the substitution.
• Dropping the power of ten, or writing 10⁻⁷ when the size should be 10⁷.
• Writing “microwaves are faster than light” or any claim that one group is faster in a vacuum.
• Describing energy transfer as “the wave carries heat” without naming the absorber and its change.
• Using the spacing of melted spots as the wavelength instead of doubling it.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), first assessment 2018, published by OCR. Topic P5.2, statements P5.2a to P5.2f.