The electromagnetic spectrum
| English | Chinese | Pinyin |
|---|---|---|
| electromagnetic wave | 电磁波 | diàn cí bō |
| spectrum | 光谱 | guāng pǔ |
| vacuum | 真空 | zhēn kōng |
| microwave | 微波 | wēi bō |
| infrared | 红外线 | hóng wài xiàn |
| ultraviolet | 紫外线 | zǐ wài xiàn |
| gamma | 伽马 | gā mǎ |
All the same family
- Radio, the heat from a fire, sunburn, a hospital X-ray — all are electromagnetic waves 电磁波.
- They differ only in wavelength (and so frequency).
- One family, one spectrum 光谱.
What they share
- All EM waves are transverse.
- In a vacuum 真空 they all travel at the same speed, $c = 3.00 \times 10^{8}\ \dfrac{\text{m}}{\text{s}}$.
The electromagnetic spectrum
Slide across the spectrum
Radio waves, visible light and gamma rays are all the same wave — only the wavelength changes, and with it the frequency, photon energy and everyday use.
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 微波 · infrared 红外线 · visible · ultraviolet 紫外线 · X-ray · gamma 伽马.
- 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 $c = f\lambda$.
Two waves of the same frequency, shifted by a phase difference
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 $c = f\lambda$