Electromagnetic Waves
| English | Chinese | Pinyin |
|---|---|---|
| electromagnetic wave | 电磁波 | diàn cí bō |
Radio, heat, light, X-rays — all the same kind of wave
- Your radio, your microwave, sunlight and a hospital X-ray seem worlds apart.
- Yet they are all the same thing: electromagnetic waves, differing only in wavelength.
- An electromagnetic wave 电磁波 is a self-sustaining ripple of electric and magnetic fields.
- Lined up by wavelength, they form the electromagnetic spectrum.
What an EM wave is
- An EM wave is a wave of oscillating electric and magnetic fields, at right angles to each other.
- It needs no medium — it travels through empty space (that's how sunlight reaches us).
- All EM waves travel at the speed of light $c$ in a vacuum ($\approx 3\times10^8\ \tfrac{\text{m}}{\text{s}}$).
- They are transverse waves, which is why light can be polarised.
In a vacuum, radio waves and X-rays travel at:
All EM waves travel at the speed of light $c$ in a vacuum.
Electromagnetic waves can travel through empty space with no medium.
EM waves need no medium — that's how sunlight crosses space to Earth.
EM waves are ____ waves, which is why light can be polarised.
EM waves are transverse (fields perpendicular to travel), so they can be polarised.
The spectrum
- In order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- Visible light is just a thin band our eyes happen to detect — red (long $\lambda$) to violet (short $\lambda$).
- Higher-frequency waves (UV, X-ray, gamma) carry more energy per photon and can be harmful.
- Lower-frequency waves (radio, micro) are gentler and carry information and heat.
Explore the spectrum
Slide across the spectrum from radio to gamma and see the wavelength and uses change.
Which has the highest frequency?
Gamma rays are at the high-frequency (short-wavelength) end of the spectrum.
Higher-frequency EM waves (like X-rays) carry more energy per photon than radio waves.
Photon energy rises with frequency, so X-rays carry more per photon than radio.
Select all true statements about electromagnetic waves.
All EM waves are transverse, need no medium, and travel at $c$. Radio is not slower than light.
Everyday uses
- Radio & micro — communications, cooking, radar.
- Infrared — heat, remote controls, night vision.
- Visible & UV — sight, sunburn, sterilisation.
- X-ray & gamma — medical imaging, cancer treatment, sterilising equipment.
All EM waves travel at the same speed $c$ in a vacuum — radio is not slower than X-rays. What differs is their wavelength and frequency (and so their energy), not their speed. Since $c = f\lambda$ is fixed, higher frequency always means shorter wavelength.
A radio wave and a light wave both travel through a vacuum. Which is faster?
- Neither — both travel at $c \approx 3\times10^8\ \tfrac{\text{m}}{\text{s}}$.
- They differ only in wavelength: the radio wave's is far longer than the light wave's.
Electromagnetic waves are transverse ripples of electric and magnetic fields that need no medium and all travel at the speed of light $c$ in a vacuum. The spectrum runs radio → gamma by increasing frequency (decreasing wavelength); higher frequency means more energy per photon.