Electromagnetic Waves
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| electromagnetic wave/ɪˌlektrəʊməɡˈnetɪk weɪv/ | sóng điện từ |
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:
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
All EM waves travel at the speed of light $c$ in a vacuum.
Electromagnetic waves can travel through empty space with no medium.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
EM waves need no medium — that's how sunlight crosses space to Earth.
EM waves are ____ waves, which is why light can be polarised.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Gamma rays are at the high-frequency (short-wavelength) end of the spectrum.
Radio waves carry more energy per photon than X-rays.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
False. Photon energy $E = hf$ rises with frequency, so high-frequency X-rays carry far more energy per photon than radio waves.
Select all true statements about electromagnetic waves.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Write from a frequency of 5 times 10 to the 7 hertz
- A radio wave has wavelength 6 m.
- Take the speed of electromagnetic waves as 3 times 10 to the 8 metres per second. Find the frequency.
A radio wave in air has wavelength 6 m. Electromagnetic waves travel at 3 times 10 to the 8 metres per second. What is the frequency of this wave?
3 times 10 to the 8 divided by 6 is 5 times 10 to the 7.
What is the frequency of the 6 m radio wave?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Wave speed divided by wavelength is 5 times 10 to the 7 hertz. Other options — 3 times 10 to the 8 hertz (the speed used as the frequency); 1 times 10 to the 8 hertz (the frequency of a 3 m wave).