Electromagnetic waves are transverse

8 minAS Level7.4.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can describe what oscillates in an electromagnetic wave, state that all electromagnetic waves are transverse, and use $c = f\lambda$ with the speed of light in free space.

Something has to be waving

A sound wave is easy to picture: bits of air move, and you can point at them. Light has no such bits. It crosses the vacuum between the Sun and your face, where there is nothing to push.

So what is oscillating? Two fields: an electric field and a magnetic field.

A field is a region where something feels a force — a charged particle feels a force from an electric field, and a magnet feels a force from a magnetic field. You do not need the full theory here. What matters is the geometry:

both fields oscillate;
they oscillate at right angles to each other;
both oscillate at right angles to the direction the wave travels.

That last point is the definition of transverse. Every electromagnetic wave is transverse.

Definition

Electromagnetic wave

A wave made of oscillating electric and magnetic fields, with the oscillations perpendicular to the direction of energy transfer.

All electromagnetic waves are transverse, and in free space (a vacuum) they all travel at the same speed:

$c = 3.00 \times 10^8\ \text{m s}^{-1}$

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves. They differ in frequency and wavelength, not in the kind of wave they are.

Same speed, different wavelengths

Because every electromagnetic wave travels at the same speed $c$ in free space, the wave equation becomes a fixed link between frequency and wavelength:

$c = f\lambda$

This is the whole reason the electromagnetic spectrum is a spectrum. A radio wave and a gamma ray are the same kind of wave; the radio wave has a long wavelength and a low frequency, and the gamma ray has a short wavelength and a high frequency. Their product is always $c$ .

The speed is only $c$ in free space. In glass, water or air, electromagnetic waves travel slightly slower, and that change of speed is what makes lenses and prisms work. The syllabus statement is about free space.

Worked example: Worked example: frequency from wavelength

A radio station broadcasts at a wavelength of $0.60\ \text{m}$ in free space.

Find the frequency of the wave.

Have a go first?

Worked example: Worked example: wavelength from frequency

An FM station broadcasts at $100\ \text{MHz}$ .

Find its wavelength in free space.

Have a go first?

Common mistakes

Radio waves must be longitudinal because sound waves are.

Sound is mechanical and longitudinal; radio is electromagnetic and transverse. Sharing the word "wave" does not make them the same kind of wave.

Light needs air to travel through.

Electromagnetic waves need no medium at all. That is why sunlight reaches us through space.

Gamma rays travel faster than radio waves in a vacuum.

In free space they all travel at exactly $c$ . Gamma rays have a higher frequency and a shorter wavelength, not a greater speed.

In the exam

Learn the statement word for word: every electromagnetic wave is transverse, and all travel at the same speed $c$ in free space. Both halves are regularly examined.
State the value as $3.00 \times 10^8\ \text{m s}^{-1}$ , and use $c = f\lambda$ rather than $v = f\lambda$ for electromagnetic waves in a vacuum.
Convert MHz and GHz before substituting. $1\ \text{GHz} = 10^9\ \text{Hz}$ .
If a question asks what is the same for all electromagnetic waves, the answer is the speed in free space — not the wavelength, the frequency or the energy.

Key takeaways

Electromagnetic waves are oscillating electric and magnetic fields, at right angles to each other and to the direction of travel.
Every electromagnetic wave is transverse.
In free space all electromagnetic waves travel at $c = 3.00 \times 10^8\ \text{m s}^{-1}$ .
Use $c = f\lambda$ : the regions of the spectrum differ in $f$ and $\lambda$ , not in speed in a vacuum.

Practice

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Apply

CalculateQuestion 3 · Apply

Multiple choiceQuestion 4 · Apply

Fill in the blanksQuestion 5 · Recall

Answer the last check to finish.