Light is a
electromagnetic radiation. We call light to the portion of the electromagnetic spectrum wich is visible to humans, from 380 to 780nm, wich is called the visible spectrum. Below it is the UV (Ultraviolet) and after it, the IR (infrared).
Plants use for photosyntesis a similar range than humans for vision. Both use mostly
wavelenghts, wich is the parameter wich identifies differents electromagnetic radiations, between 420 and 685nm, although photosynthesis still take place at noticiable amounts from 400 to 700nm, wich is many times used too as a simplified visible range, due light below 400nm and over 700nm are very little visible.
This range, 400-700nm, is called the PAR (Photosynthetic Active Radiation) range. Very often, although is stated the opposite, when we talk about light, we are refering to the PAR range.
A
nanometer (nm) is a 10^(-7) meters=1/10000000 m=0.0000001m.
Visible range is divided into the different pure colors:
Violet:380-430nm
Blue:431-480nm
Cyan:481-510nm
Green:511-565nm
Yellow:566-590nm
Orange:591-625nm
Red:626-780nm (its often splitted into near red (or just red), up to 700nm and far red, over 700nm)
(Note: Exact limits between colors arnt clearly defined, so you can find other limits for these colors, but its, give or take, correct.)
A very relevant caracteristic of electromagnetic radiations, thus of light, is it behalf both as a wave and as particle. This is called "
wave-particle duality". In the practice, this mean we can understand light in any of both ways at our convenience. So when we talk about optic watts of light, we are taking it as wave, and when we talk about photons, we are treating it as particle.
Generally, light is treated as wave when we study how it propagates and behave alone, but as particle (photon) when we study how it interact with materia (as when lights hit the plant).
There is an important practical consecuence of using one or the other: energy wich carries a photon is inversely proportional to its wavelenght (wl). So a mol of photons will carry more energy as shorter is its wl, or the inverse: a watt of light carries more photons as longer is its wl.
Plants dont know nothing about watts, they sense and use light from the amount of photons they receive, independent of its wl. This is the deep cause behind plant uses better red photons (longer wavelenghts) than others of shorter wl. And this is the reason botanist uses the number of photons to study how light affect plants, instead of watts.
The unit used to measure the number of photons is the
mole, wich equal aprox to 6.02*10^23 particles (Avogadro's number). Most times, its used its million diminutive, the micromol, wich is 1/1000000 mole. For light, is commonly abreviated to E (Einstein) or μE (microEinstein, often written uE for keyboard limitations). Although it isnt an official unit, ill use them for confort.
As reference, a 400w HPS emits about 650 uE (per second) (in PAR, always its not stated in other way).