wavelength and frequency

Picture a duck bobbing on a calm lake while ripples roll past it. Two simple questions describe those ripples. How far apart are the crests? That distance is the wavelength. How many crests pass the duck each second? That count is the frequency. Every wave, including every kind of light, can be described by these same two numbers, and once you know one you can find the other.

For light, the wavelength (often written with the Greek letter lambda) and the frequency (often the Greek letter nu) are tied together by a single rule: wavelength times frequency equals the speed of light, which is the same for all light. Because their product is fixed, they trade off: a long wavelength means a low frequency, a short wavelength means a high frequency. Wavelengths span an astonishing range, from kilometers for radio waves down to less than the size of an atomic nucleus for gamma rays. We measure visible light in nanometers (billionths of a meter); red light is about 700 nm, violet about 400 nm.

These two numbers are the practical handle astronomers use to describe and sort light. The same band can be labeled by either: radio astronomers usually quote frequencies (megahertz, gigahertz), while optical and infrared astronomers usually quote wavelengths (nanometers, microns). Frequency also connects directly to a photon's energy (energy equals Planck's constant times frequency), so 'higher frequency,' 'shorter wavelength,' and 'more energetic' all mean the same thing. When light from a distant galaxy is stretched on its way to us, it is exactly the wavelength that grows and the frequency that drops — the basis of redshift.