How important is vertical light?

[BeefyG]

I'm using a clamp on flexible shaft light with a cfl bulb to light a small 10 gallon planted tank. The stand I'm using is an old computer desk and the tank is underneath the keyboard tray. I probably could push the tank further back and shine the light straight down if needed. Right now its at maybe a downward 45 degree angle. Is there a reason aquarium lights always shine down from the top or is it just aesthetics? I understand its more natural that way but then again the sun moves all day long and at some point replicates a 45 degree angle. What do ya'll think?

[myjohnson]

If you are growing plants then your plants will grow towards the light.

Fishes will not be effected one way for the other.

[Mike B]

I have had a 46 gallon freshwater community tank running for over a year with a 20 watt 24" under cabinet flourescent fixture laying on the back of the stand at gravel level backlighting the tank. The fish don't seem to mind one bit and it looks good at night. When I try my brown thumb at changing it over to real plants I will add more tubes and move the light source overhead.
Mike B

[Mzungu]

For some reason I remember reading something about how light from above is beneficial to fishes because it helps them orient themselves. As in, if lighted from the side they'll tend to list a little sideways in order to have their backs parallel to the light source.

I have no idea from where I remember that, lol. I'll look around and see if I can't find it again.

But yes, like Thai said, the plants will grow towards the light, so if your light is coming in at an angle, the plants will lean that way.

[Mzungu]

Evidently it matters to copepods!

http://www.biolbull.org/cgi/reprint/135/1/239.pdf

[myjohnson]

Wow, good read MB!

Thanks for the link.

[kwc1974]

+1 nice read MB
+1 light for plants should come from above.

[Gapzero]

I know that light angle matters to some reptiles. To get them to breed you have to use three lights, one on each side and one in the middle. then use timers so the lights go on and off in pattern that replicates the sun rising and setting. Their might be a few shallow water fish that would breed easier with the same technique.

[Mzungu]

Thanks, guys! I love those journal articles written before 1950, they don't use words I don't understand, lol.
I spent about a half hour after finding that article looking for more, and made some progress, but then I realized I needed to get to work. I hope to find something, though, that talks about exactly that question - whether it disorients fish to have light coming from places other than above. It apparently confuses predators, though, I learned this morning.

I know I could google it, but if someone could explain what the term "polarization" and/or "polarized light" means before I get home tonight, that would be awesome. 8)

[Mzungu]

BTW, great question, BeefyG!!

[Gapzero]

Light consists of oscillating electrical fields. Light whose electrical field oscillates in a particular way is said to be polarized. The fields can oscillate on a plane, or from left or right, or circularly down like a screw. There is no such thing as unpolarized light , all light has the fields. It usually refers to light with random oscillations, coming from all angles. Not sure how deep you want to go. Do you want to know how to polarize light or how light waves work?

[Mzungu]

Well, it's just that a lot of the articles I was looking at this morning were talking about the effect of polarized light on fish, and the e-vector angle, or something like that. So I wasn't sure if "polarized" light coming down into the water was somehow different or modified from plain old sunlight.

Thanks for the explanation!

[Gapzero]

It really depends on your light. Usually the light bars on fish tanks are not really polarized at an angle. Also any light that is reflected back into the tank via the metal reflector is also unpolarized as any light reflected off metal loses its "polarization". You have to build a polarizer to directly polarize your water into the tank. I think most fish do not live by direct or polar light, but by ambient light. Plants grow toward light, but fish that live at certain depths do not go shallower if there section gets to dark( from algae or plant growth at the surface) except if necessary to follow food.