Visible light is a wave too, but it needs no medium: it is a wave of electric and magnetic fields, and it crosses empty space at $300{,}000$ km/s. Visible light is only a thin slice of a much larger family. Every member obeys the same wave equation $v = f\lambda$, all with the same speed in space.
Key idea
The electromagnetic spectrum, from longest wavelength to shortest: radio waves, microwaves, infrared, visible light (red to violet), ultraviolet, X-rays, gamma rays. All of them are light, and all travel at about $3 \times 10^8$ m/s in empty space.
Why it works
Since every kind travels at the same speed $c$, the equation $c = f\lambda$ means a longer wavelength comes with a lower frequency. Radio waves can be longer than a building, with low frequencies; gamma rays are smaller than an atom, with enormous frequencies. Visible light runs from about 700 nm (red) to about 400 nm (violet), where 1 nm is a billionth of a meter.
Worked example: Order the bands
Put these in order from longest to shortest wavelength: X-rays, radio waves, visible light, microwaves.
- Use the spectrum order: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- So: radio waves, microwaves, visible light, X-rays.
Answer: Radio, microwaves, visible light, X-rays
Worked example: Moonlight's trip
The Moon is about 384,000 km away. Light travels at about 300,000 km/s. How long does moonlight take to reach us?
- $t = \dfrac{d}{v} = \dfrac{384{,}000}{300{,}000} = 1.28$ s.
Answer: About 1.3 s
Common mistake
Thinking radio waves are a kind of sound, or that only visible light is "light." Radio waves are electromagnetic waves, like light, that travel through space at the speed of light; a radio turns them into sound only at the end. Sound itself can't cross empty space at all.