Can someone explain to me about mirrors as light reflectors?

[bleedingthought]

How do they absorb light? And do they absorb all spectrum of light? And why is it that they're not as good of a reflector as polished aluminum, for example?

I have heard/read that a smooth surface that is painted with glossy white paint is supposedly more reflective than mirrors. Yet, as far as our eyes are concerned, looking into a mirror that's reflecting a bright light is a lot "brighter" than looking at a flat white surface.

Thanks in advance! I've read a couple of articles but they're a little over my head.

 

[Anonymous]

Mirror absorb light from the glass that the light need to pass thru on the way to the silver/aluminum coating on the back, and back up. The silver/aluminum also does not reflect light 100%, so there is a lost there too.

Mirror is good specular reflector. A white surface does not give you a specular surface, but a lot of light is reflected back, just in all different direction. If you aim a mirror incorrectly, you won't see the light. But a white, glossy surface is more forgiving.

 

[bleedingthought]

Ah, I see. So, if you used a mirror as a light reflector for a reef tank, for example, then it would be more of a 'topical' reflector? As in, it would only reflect the light back to a certain spot, right? (Assuming it's a single piece of a mirror in a flat surface, and not a bunch of little mirrors at different angles)

 

[bleedingthought]

I should elaborate more. There is a fellow reefer in the club that wants to do a bare bottom tank with mirrors on the bottom to reflect light back up and under the corals. Would it be more efficient to use mirrors or white starboard?

 

[Anonymous]

Under the sea, all the light hits the coral from within the Snell'scone, which is about 40 degree. It is not natural for direct light to hit the underside of the coral.

By using the mirror on the bottom, he can see which part is not illustrated by the reflection, and may want to change the mirror to the starboard, where there is less likely to have missed area under the coral.

 

[bleedingthought]

Ok, starting to make sense now. So, about how light DOES a mirror reflect?

 

[pcardone]

I once read that white reflects a high percentage

 

[Anonymous]

White reflects almost as much as mirror, surprisingly. But the main reason you want a good reflector is that shimmering (caustic in optics) is more dramatic with mirror. For sand bottom, it does not matter that much.

Even completely black surfact reflect light. In shiny surface, such as a factory-fresh car that is painted blackest black, you can still see the sun off the paint. For rough surface, it still reflects a bit of visible light.

(btw, I missed a space on "Snell's cone")

 

[Anonymous]

a mirror reflects 88 to 93% of the incoming light according to the CRC handbook.

The problem with putting any reflector on the bottom (or under) the tank is that eventually it will get covered in coraline algae, and stuff there is a lot harder to scrape off than off the walls of the aquarium. I'd suggest not doing anything to reflect light up, use starboard if he needs some impact resistance and that's it.

 

[Anonymous]

>...a mirror reflects 88 to 93% of the incoming light according to the CRC handbook.

Get some spacecraft-grade reflector that have reflectivity of 103%! Oh, wait....

 

[Anonymous]

Why not have the back panel of the tank be a mirror. It would make the tank look bigger.

FWIW, I use reflective mylar underneath my hood.

 

[Anonymous]

Why not have the back panel of the tank be a mirror. It would make the tank look bigger.

And the fish would be extremely po'd or get stressed out whenever they saw someone that looked sort of like them that was getting close to their territory.

Then the coraline comes in and ruins your mirror idea

 

[bleedingthought]

See, the fellow that wants to use the mirrors in the bottom of the tank already knows he will be faced with coralline. He says he wants to try and keep it clean.

But that wasn't my beef with it. People just started to say, "wow, mirrors as reflectors are a great idea!" and I started telling them that as far as I know, mirrors aren't good reflectors and I thought they also absorbed a great deal of light.

 

[Anonymous]

Well what I would suggest is instead of mirrors, try to find some reflective mylar (better than mirrors) put it under the tank, that way you have something really paper thin and don't have to worry about any sort of thickness issues you'd have with mirrors. Unless he just has a ton of those 12" x 12" mirrors lying around and wants to toss them under. Then go for it.

 

[Anonymous]

I don't think mylar is as good of a reflector as mirror... you can see thru it if you put your eyes close enough.

 

[Anonymous]

95% reflectivity can't be wrong

 

[Anonymous]

95? 8O

 

[Anonymous]

Yup. 95.