[APD] RE: Aquatic-Plants Digest, Vol 4, Issue 11

["Jim Seidman" <js5 at seidman_net>]

George Booth had written:
> I had a chance to measure the lighting in a bare 29g tank
> once with a
> luxmeter. With water, of course. I found that the light
> at lower levels
> was actually a little more intense than that just below
> the surface! I
> checked that a couple of time since it was so
> counter-intuitive.

And S. Hieber replied:
>I'd expect that unless you measured directly under a bulb
>both times and there was only one bulb. Otherwise, I'd
>expect that you'd get the light from the others bulbs, if
>there are more, if you go down a bit lower.

Even with one bulb I could see those results, because of the way that light
reacts when encountering media with different indices of refraction.

I'm sure someone will correct me if my memory of this is just too fuzzy (i.e.
totally wrong). But the following is what I would expect to happen.

The light coming from the bulb will hit the water at a variety of angles. When
it enters the water, the angle will become closer to vertical. This is one big
reason why less light leaks out of a full tank than an empty one - the light in
the full tank is going in a direction closer to straight down.

Even with this reduction in angle, a fair amount of light is going to be
hitting the sides of the tank. There's enough of a difference in the indices of
refraction between glass and water (about 1.5 and 1.3, I think) that much of
the light will reflect back into the tank. An additional chunk of light will
bounce off of the glass-air interface into the tank.

This means that near the surface, a luxmeter would only get light directly from
the bulb. Lower down, you would get light both directly from the bulb and
reflected off of the glass.

This would have much more of an impact near the sides of the tank than directly
in the middle, though.

- Jim


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