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The Electromagnetic Spectrum
In the radio part of the spectrum both are used. The long wave 'Radio4' is talked about as a
frequency of 198KHz (or 198,000 waves per second) or a wavelength of 1500metres (where the distance
between successive wave peaks is 1.5 kilometres). With light, the frequency of the waves is so very high that it is an inconvenient measure, their wavelength is very short, but at least we have convenient numbers for it, and thus light is usually talked about as its wavelength. Green light for example has a wavelength of 555 nanometres (0.000000555metres) and its frequency is a number with 16 zeroes after it!
Human eyes have their peak sensitivity at 555nm, which is in the green part of the spectrum. (remember ROYGBIV - Richard of York gave battle in vain - gives the rainbow spectrum order of colour and decreasing wavelength, i.e. red/orange/yellow/green/blue/indigo/violet). The sun produces a continuous spectrum received at the earth's surface from ultraviolet to radiowaves, the strongest and most significant being the visible and infrared regions. A tungsten light bulb will produce visible and infrared energies. A fluorescent tube will produce portions of the visible spectrum but very little of the infrared region. Many other lamps are available, some giving you more useful energy (to plants) per watt of electricity required to power them than others.
See the representation of the Electromagnetic Spectrum below to see where our visible range fits into the complete picture.
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