Electromagnetic Waves: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 13 · Read time: ~7 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Properties shared by all EM waves
All electromagnetic waves are transverse waves, can travel through a vacuum (no medium needed), and travel at the same speed in a vacuum, 3.0 × 10⁸ m/s.
They all obey the wave equation v = fλ and transfer energy. They can be reflected, refracted and diffracted, just like the light waves covered in Chapter 9.
Going from radio waves to gamma rays, frequency increases and wavelength decreases. Higher frequency also means more energy carried, which is why the high-frequency end is the most dangerous.
Trap: Sound is not an electromagnetic wave. It's a longitudinal wave that needs a medium and cannot travel through a vacuum. MCQ options often slip sound or ultrasound in alongside the EM regions to catch this.
Uses of each region
Radio waves: radio and television broadcasting, long-distance communication. Microwaves: satellite and mobile phone communication, microwave ovens.
Infrared: remote controls, thermal imaging cameras, intruder alarms, toasters and grills. Visible light: seeing, photography, optical fibres for communication.
Ultraviolet: sterilising water and equipment, detecting forged banknotes, sunbeds. X-rays: medical imaging of bones, airport baggage security. Gamma rays: radiotherapy to kill cancer cells, sterilising medical equipment and food.
Trick: Remember the order with a mnemonic: 'Raging Martians Invaded Venus Using X-ray Guns' (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma). It runs in order of increasing frequency.
Dangers of EM radiation
Microwaves can heat body tissue internally. Infrared in large amounts causes burns.
Ultraviolet damages skin cells and eyes, and can lead to skin cancer and sunburn, which is why sunscreen and UV-blocking sunglasses matter.
X-rays and gamma rays are ionising: they can damage or kill living cells and cause cancer. Radiographers stand behind lead screens and limit exposure time for this reason.
Trap: Don't write that all EM waves are harmful in the same way. Exam answers need the specific danger for the specific region, for example 'UV can cause skin cancer', not just 'it is dangerous'.
Worked Example
A local radio station broadcasts at a frequency of 95.0 MHz. The speed of electromagnetic waves in air is 3.0 × 10⁸ m/s.
(a) Calculate the wavelength of the radio waves. [2]
λ = v / f
= 3.0 × 10⁸ ÷ 95.0 × 10⁶
= 3.2 m (2 s.f.)
(b) A microwave oven uses microwaves of wavelength 0.12 m. Calculate their frequency. [2]
f = v / λ
= 3.0 × 10⁸ ÷ 0.12
= 2.5 × 10⁹ Hz
(c) State one property that the radio waves and microwaves share, and one way in which they differ. [2]
Shared: both travel at 3.0 × 10⁸ m/s in a vacuum (or: both are transverse, both can travel through a vacuum).
Differ: microwaves have a higher frequency and shorter wavelength than radio waves.
Why this question is a good test of the topic: it checks unit prefixes (MHz means × 10⁶) as much as the formula. Forgetting to convert MHz to Hz is the single most common way to lose both marks in part (a).
Frequently Asked Questions
What is the order of the electromagnetic spectrum?
In order of increasing frequency (and decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Do all electromagnetic waves travel at the same speed?
Yes, in a vacuum they all travel at 3.0 × 10⁸ m/s. They slow down by different amounts in materials like glass or water.
Is sound part of the electromagnetic spectrum?
No. Sound is a longitudinal wave that needs a medium. Electromagnetic waves are transverse and can travel through a vacuum.
Which EM waves are ionising?
X-rays and gamma rays are ionising and can damage living cells. High-energy ultraviolet can also damage cells and cause skin cancer.
What are infrared waves used for?
Remote controls, thermal imaging cameras, intruder alarms, and cooking in toasters and grills.
Need help remembering every region and its use?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author, with a team of tutors.