Larnek said:
As for penetration I'd have to hazard and guess that light from a larger light source would have photons of a higher energy level (quanta) and so give more light to those leaves beneath the canopy. If I can remember my hated chemistry right photons have different energy levels corresponding to amount of electrons that can be knocked free. This is dependent on the amount of energy that created said photons as well as the wavelength in which is travels. This is where it gets tricky as you have to delve into quantum physics where light becomes a particle as well as a wave and is seriously wacky and more in depth than I want to remember. Basically the photon (particle) of light strike the chloroplast and gives energy to electrons in the chloroplasts freeing them from their bonds. The number of electrons knocked off would be equivalent to the number of electrons the chloroplasts are able to move to the electron chain which feeds the carbon cycle. This part is true, I even went back and grabbed my old notes! What I don't know for sure is whether or not a 1000w light actually creates photons of a high energy level or just more photons on a whole.
There are two things you need to consider when dealing with photon beams: intensity and energy. The intensity concerns
how many photons per unit area are striking the plant's leaves. The energy deals with the energy of
each individual photon. Think of intensity like traffic on a freeway (light or heavy) and energy like the speed of the cars (fast or slow).
Intensity
With bulbs, intensity is the strength or power output. It's basically how many lumens the bulb emits. A 2000 lumen bulb will do the same job as two 1000 lumen bulbs.
Energy
With bulbs, energy is the spectrum of the light. Ever heard of warm or white or sunlight or (insert description of light) bulbs? These depend on the energy of the light. Sometimes you will see a temperature on the bulb package, like 4000K, 5500K, or 6500K. (The K stands for Kelvin, the internationally accepted scientific standard for temperature. To convert to Celsius just subtract 273.) Obviously the light bulb isn't actually getting that hot; this just denotes the spectrum of energy that would typically be emitted by black body radiation at that temperature. Hint: the sun can be considered a black body. Wikipedia says the sun's surface temperature is 5778K and that's why if you're using fluorescents, many suggest getting 5500K bulbs. That is, they mimic the sun's light spectrum. Two 2500K bulbs
WILL NOT be the same as one 5000K bulb.
If you remember the first equations they taught in Physics, energy is dependent on the wavelength of light. Since light bulbs emit many wavelengths it's appropriate to talk about the spectrum of wavelengths. But as
Larnek pointed out, light is best described using a wave-particle duality theory. That's why you can think about discrete photons (particle) that have a wavelength (wave). There are some really neat experiments that show how light
simultaneously acts as both.
Photosynthesis
The electrons everyone's concerned with are in the reaction center of the chlorophyll. They are highly-conjugated systems that have evolved to have the same energy gaps that correspond with the strongest portions of sunlight's energy spectrum. Basically what happens is photons, either from the sun or your bulbs, hits the pigments in the reaction center of clorophyll. These pigments have their electrons excited, redox reactions occur, etc ad nauseum.
Putting it all together, a stronger bulb will produce more photons. More photons hitting the leaf will cause photosynthesis to proceed faster and the plants will grow faster. A stronger bulb will emit approximately the same spectrum as a weaker bulb.
One might ask if a bulb with more energy would help. If a bulb was used with a spectrum that had more high-energy wavelengths than low-energy wavelengths, what effect would that have?
Think about it. The plant's pigments have spent millions of years evolving to match the sun's spectrum as best as possible. If the sun's spectrum peaks at XXX nm, YYY nm, and ZZZ nm (nm=nanometer, a convenient unit of measure for wavelengths), then the top three pigments in the chlorophyll are best excited by XXX nm, YYY nm, and ZZZ nm light. So a higher-energy bulb isn't necessarily what you want. The farther you shift your bulb's spectrum from that of sunlight, the less energy the pigments can absorb. The thing about quantum mechanics is that energy levels are discrete; you can't just pump in any amount of energy. The electrons need discrete quanta and giving it
too much energy (like UV light) won't help.
If you've ever thought about LED lights, they work by emitting a very small range of wavelengths. A light bulb that encompasses all visible light will have wavelengths of 400 nm to 750 nm with portions at higher (infra-red) wavelengths, too. LEDs only deliver a few Watts of light but they invest it all in one (in reality, a small range) wavelength. That's why they always have a color; blue LEDs emit 450-460 nm. Their wattage is low so the intensity is low; few photons are being emitted compared with other bulbs. But the energy is tweaked the match precisely the energy pigment electrons need to enter photosynthesis. So the argument for LEDs is that they use less power but more of it is absorbed by the plant. (I understand the many arguments but I won't present them here. I'm just trying to apply quantum mechanics to LEDs as an example.
This whole reply was basically a lesson on the symantecs of intensity and energy. Gotta go harvest now!
