[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Various comments on lighting

Neil Frank asked:

> Wright: is the light coming back into the room from reflected surfaces like
> the gravel?


The rule of optics that says what gets out of the tank is governed by
something called the "critical angle," and any light hitting the glass at an
angle less than critical can exit into the room. [Assumes 0 degrees is
normal or perpendicular to the glass.]

For water and glass at an index of refraction near 1.5, the critical angle
is about 45 degrees. Light hitting the inside of the glass-air surface at
the critical angle is bent (refracted) to propagate parallel to the glass
surface, and anything at greater angle suffers total internal reflection.
The reverse is true at the water air interface above. Horizontal light is
bent down to 45 degrees inside the tank, hence it can't get out through the
glass until redirected.

Clean water has negligible scatter in the visible, so the prime sources
(assuming a normal fluorescent hood) are the gravel and the silicone seams
in a bare tank. Ripples on the water surface can let tiny amounts enter from
above at angles very close to but below critical, so you can see some faint
ripples of light on the floor when a filter disturbs the surface. Looking up
the side glass, though, you can't actually clearly see the bulbs and hood
from outside the tank. A clean, still water surface just looks black.

Once light propagating down through the tank hits anything that reflects or
scatters it more toward the side glass, that reflected light *can* leave the
tank. All the light must eventually either leave that way, or be absorbed by
objects less than 100% reflective -- hopefully plants in our case.

Hope that helps more than confuses.


Wright Huntley, Fremont CA, USA, 510 494-8679  huntleyone at home dot com

           To err is human. To blame someone else is politics.
               *** http://www.self-gov.org/index.html ***