The electromagnetic spectrum
All the same family
- Radio, the heat from a fire, sunburn, a hospital X-ray — all are electromagnetic waves.
- They differ only in wavelength (and so frequency).
- One family, one spectrum.
What they share
- All EM waves are transverse.
- In a vacuum they all travel at the same speed, $c = 3.00 \times 10^{8}\ \dfrac{\text{m}}{\text{s}}$.
All electromagnetic waves are transverse.
Yes — every EM wave is transverse, and all travel at $c$ in a vacuum.
In a vacuum, which EM wave travels the fastest?
In a vacuum every EM wave travels at $c = 3.0 \times 10^{8}\ \dfrac{\text{m}}{\text{s}}$.
The spectrum in order
- From long to short wavelength: radio · microwave · infrared · visible · ultraviolet · X-ray · gamma.
- As wavelength decreases, frequency increases.
Put these in order of increasing frequency (lowest first).
Frequency rises as wavelength falls: radio (longest) → microwave → visible → X-ray (much shorter).
Wavelengths and the link
- Learn the orders of magnitude — e.g. radio metres, X-rays $\sim 10^{-10}\ \text{m}$.
- Visible light is only $400$–$700\ \text{nm}$. Switch between $\lambda$ and $f$ with $c = f\lambda$.
Visible light has wavelengths of roughly:
About $400\ \text{nm}$ (violet) to $700\ \text{nm}$ (red) — a tiny slice of the whole spectrum.
A radio wave has wavelength $3.0\ \text{m}$. What is its frequency, in MHz? (Use $c = 3.0 \times 10^{8}\ \dfrac{\text{m}}{\text{s}}$.)
$f = \dfrac{c}{\lambda} = \dfrac{3.0 \times 10^{8}}{3.0} = 1.0 \times 10^{8}\ \text{Hz} = 100\ \text{MHz}$.
You've got it
- all EM waves are transverse and travel at $c = 3.0 \times 10^{8}\ \dfrac{\text{m}}{\text{s}}$ in a vacuum
- order: radio → microwave → IR → visible → UV → X-ray → gamma
- visible light is $400$–$700\ \text{nm}$; use $c = f\lambda$