The electromagnetic spectrum Cambridge International AS & A Level Physics revision
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In plain words
Radio waves, the warmth from a fire, light, X-rays: they are all the same kind of wave, differing only in wavelength. Every one of them is transverse, and every one crosses empty space at the same speed.
4 things to know
- All electromagnetic waves are transverse and travel at the same speed in free space: c = 3.00 × 10⁸ m/s.
- In order of decreasing wavelength, with rough ranges: radio waves (longer than 10⁻¹ m), microwaves (10⁻¹ to 10⁻³ m), infrared (10⁻³ to 7 × 10⁻⁷ m), visible light (7 × 10⁻⁷ to 4 × 10⁻⁷ m), ultraviolet (4 × 10⁻⁷ to 10⁻⁸ m), X-rays (10⁻⁸ to 10⁻¹³ m) and gamma rays (10⁻¹⁰ to 10⁻¹⁶ m).
- Visible light has wavelengths from about 400 nm (violet) to 700 nm (red) in free space.
- Because c = fλ and c is the same for all of them, a shorter wavelength means a higher frequency.
Worked example
Find the frequency of microwaves of wavelength 3.0 cm in free space.
- c = fλ, so f = c ÷ λ.
- = 3.00 × 10⁸ ÷ 0.030.
- = 1.0 × 10¹⁰ Hz.
Tips and tricks
- X-rays and gamma rays overlap in wavelength. They are told apart by how they are made, not by their wavelength.
- Learn one typical wavelength for each region: radio about a metre and up, microwaves about a centimetre, visible about half a micrometre, X-rays about a tenth of a nanometre.
It lands in your notebook with its questions as flashcards.
The electromagnetic spectrum: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the speed of all electromagnetic waves in free space?
This is c.
Which has the shortest wavelength?
And so the highest frequency.
Which region of the spectrum has wavelengths of about 1 cm?
Microwaves run from about 10⁻¹ m down to 10⁻³ m.
Which has the higher frequency, red light or violet light?
Violet has the shorter wavelength.
A wave has a wavelength of 5 × 10⁻⁷ m in free space. Which region is it in?
That is 500 nm, between 400 nm and 700 nm.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 5 marks. Write your answers on paper, then check them.
The electromagnetic spectrum
Cambridge International AS & A Level Physics 9702 · 5 marks · papermunch.org
Name ______________________________ Date ______________
Find the wavelength in free space of radio waves of frequency 100 MHz.[2]
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3.0 m.
State two properties shared by all electromagnetic waves.[2]
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They are all transverse waves, and they all travel at the same speed in free space (3.00 × 10⁸ m/s).
State the range of wavelengths of visible light in free space.[1]
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About 400 nm to 700 nm.
Answers: The electromagnetic spectrum
- 1. 3.0 m.
- 2. They are all transverse waves, and they all travel at the same speed in free space (3.00 × 10⁸ m/s).
- 3. About 400 nm to 700 nm.