Order the electromagnetic spectrum
Recall the principal electromagnetic regions and compare their wavelength and frequency in free space.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 7. These original wave examples and diagrams are conceptual study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use frequency, period, wavelength, amplitude, speed and trigonometric ratios with consistent units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How do electromagnetic waves travel in free space?
What you will learn
- State that all electromagnetic waves are transverse and travel at c in free space
- Order principal spectrum regions from radio to gamma by wavelength or frequency
- Recall the visible wavelength range of 400–700 nm
All electromagnetic waves share key properties in free space
Electromagnetic waves are transverse: their electric and magnetic field oscillations are perpendicular to the direction of propagation. In free space, every region of the electromagnetic spectrum travels at the same speed c.
The principal regions from lower frequency and longer wavelength to higher frequency and shorter wavelength are radio, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. Their names identify ranges along one continuous spectrum.
Use inverse frequency-wavelength ordering
For waves in free space, c = fλ. Since c is constant, a greater frequency corresponds to a shorter wavelength. State the direction of the ordering whenever you list regions to avoid reversing the relationship.
Visible light has wavelengths approximately 400–700 nm in free space. This range lies between infrared at longer wavelengths and ultraviolet at shorter wavelengths.
Pause and explain
What is the approximate visible wavelength range in free space?
Worked example
Which region has the shorter wavelength: infrared or ultraviolet? Where does visible light lie relative to them?
Show the worked solution
- The order toward shorter wavelength is infrared, visible, ultraviolet.
- Therefore ultraviolet has shorter wavelength than infrared.
- Visible lies between the two regions.
Answer Ultraviolet has the shorter wavelength; visible light lies between infrared and ultraviolet.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the principal electromagnetic regions in order of increasing frequency.
Give me a hint
Start at the long-wavelength, low-frequency end.
Compare my reasoning
- Begin with radio and microwaves.
- Continue infrared, visible, ultraviolet and X-rays.
- Gamma rays have the highest frequency in this listed order.
Radio → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays.
Look for these in your work
- All seven regions included
- Correct increasing-frequency order
- Ordering direction stated
A free-space electromagnetic wave has frequency 6.0 × 10¹⁴ Hz. Use c = 3.0 × 10⁸ m s⁻¹ to find its wavelength and identify whether it lies in the stated visible range.
Give me a hint
Use λ = c/f and compare with 400–700 nm.
Compare my reasoning
- λ = (3.0 × 10⁸)/(6.0 × 10¹⁴) m.
- λ = 5.0 × 10⁻⁷ m = 500 nm.
- Compare 500 nm with the visible interval.
500 nm, within the approximate 400–700 nm visible range.
Look for these in your work
- Wave equation rearranged
- Metres converted to nanometres
- Visible range checked
An electromagnetic wave's frequency increases while it remains in free space. State what happens to its wavelength and speed.
Give me a hint
Use c = fλ with c constant in free space.
Compare my reasoning
- The speed remains c in free space.
- For c = fλ to remain constant, wavelength decreases as frequency increases.
- The wave remains transverse.
Its wavelength decreases, while its speed remains c in free space; it is still transverse.
Look for these in your work
- Constant free-space speed stated
- Inverse frequency-wavelength relation used
- Transverse property retained
Common mistakes
- Reversing the frequency and wavelength trends
- Saying electromagnetic waves require a material medium
- Giving the visible range in metres without the requested scale
What can you explain now?
How do electromagnetic waves travel in free space?
Compare with the explanation
As transverse waves at speed c
The spectrum regions share this free-space speed and transverse nature.
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Make it stick
Why do higher-frequency electromagnetic waves have shorter wavelengths in free space?
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