# [APD] Re: myths/light

George has it right in the last line, tanks with reflectors are a whole new story. We have lots of smart people on this list, but the light intensity discussion continues to crop up from time to time.

A single point source has a intensity profile that falls off as 1 over the square of the distance (1/r^2, inverse square law) Simple reasoning is the light is still providing the same amount of energy, but it is spread out over a greater area at longer distances, hence less intensity.

A linear source as George pointed out, falls off as 1 over the distance (1/r), but only for a infinite linear source.

Reflectors will induce loss. I cannot think of any optical component that provides 100% reflectance. Each bounce does incur loss. But the amount for a good reflector is negligible for the benefit of redirecting all the energy that otherwise would have been lost radiating upward into the canopy.

If we really wanted to get ugly, we could do the modelling and calculate what the intensity profile would look like as a function of position within the tank, taking into account the geometry of a reflector, how much loss the reflector induces, water absorption, reflections from the water surface, internal reflection from the glass walls etc, model it to the nth degree.

But to what end? Either plants grow well within your setup for the conditions you have provided, or they dont, and we adjust tank parameters, plant selection, and nutrient levels accordingly.

```Message: 1
Date: Tue, 7 Sep 2004 08:42:06 -0700 (PDT)
From: "S. Hieber" <shieber at yahoo_com>
Subject: Re: [APD] Re:myths/light
To: aquatic plants digest <aquatic-plants at actwin_com>```

```Doth not reflected light scatter too? And each bounce
yields less light, no?```

```Scott H.
--- gbooth at frii_com wrote:```

```> > Date: Sun, 5 Sep 2004 11:16:38 -0700 (GMT-07:00)
> . . . Linear sources such as
> flourescent bulbs
> produce illumination that drops of inversely to distance
> (twice as far
> away, half as bright).  Also remember that this is in
> "free space" without
> reflectors.  Light with reflectors over a tank of water
> is a whole
> different animal.```

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