In article ,
Jeffrey Barker wrote:
"Tim B" wrote in message ...
Things you plant underground can't tell if there's light or not until
they're on top of the ground. Lettuce, planted on top of the ground,
needs light ... not massive amounts, but enough light to set off the process
that says, hey it's spring, let's grow. Once you have two true leaves,
everything you started will need light, preferably right on top of it 1-3
inches away so the plants don't go stretching toward the light.


Ok. This is not a troll, and I really hope it doesn't seem that way.
I'm planning on getting a grow light for the first time this season,
after doing some mild indoor and outdoor gardening for a few years.

The whole thing about putting the light so close seems a bit
confusing. (I'd like to use just one or two lights for all of my
plants so I don't have to spend a fortune on lights.) The SUN is
pretty far away, so why is it such an issue to have the lights so
close to the plants? I know the answer is so they don't stretch out
and get spindly, but why does artificial light have that effect? I
know this must seem like a really idiotic question, but none of my
gardening friends seem to know the answer.

Light falls off with distance from the source. How much depends
on particulars. If we approximate a fluorescent tube as a "line"
source of light (radiating equally from the line), then we can say
that the light intensity falls as 1/d (d = distance from the line source
to some point. In my "hot box" I cheat -- since this is not in a
window-sill, I simply cover the interior of the box with aluminum foil.
This essentially eliminates the 1/d falloff, so I can keep the lights
in the same place as the plants grow. If that isn't done, we would
expect that the light intensity will be about 12 times weaker at 1'
distance than at 1" distance (of course at 1" distance the line approximation
isn't particularly good).

The sun (from our perspective) is a point source; light intensity falls
off with 1/r^2 from a point source, considerably faster than from a
line source. However since we are approx 93*10^6 miles from the sun,
a 1 foot movement away from the sun (ignoring atmospheric absorption
or other attenuation, or reflections) will result in an attenuation of
only 1 part in about 2*10^26 (!). Yes, the effects that we've ignored
will have a much stronger effect than the one we were examining.

Hey, wasn't that fun?

-frank
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