# The mirrored sphere

## Main Question or Discussion Point

Well I was thinking and I came up with a question that stumped me so I come to you for some insight.

Say you had a glass sphere with a reflective surface on the inside (kinda like interigation mirror where you can see through one side but not the other), what would you see if you looked into it? would it be black, or white, or, well I don't really know.

Edit: if it did capture light, could it build up and.. do something?

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Danger
Gold Member
I assume you mean that you have a theoretical 2-way mirror that's totally reflective on the inside. That can't exist, but if it did you would see nothing.

I can't tell what he means. I think he might mean a sphere that is a one way mirror with a perfect reflective surface on the inside. Which also can't exist. But if it did you would still see nothing. Different slant. Same diff.

I can't tell what he means. I think he might mean a sphere that is a one way mirror with a perfect reflective surface on the inside. Which also can't exist. But if it did you would still see nothing. Different slant. Same diff.
You can take a flashlight inside.

Defennder
Homework Helper
For those having problems visualising, imagine you're trapped in a sphere, and the curve walls which form the inside of the sphere are completely reflective (made of silver). You have a flashlight and you turn it on. What will you see?

Andy Resnick
Danger
Gold Member
For those having problems visualising, imagine you're trapped in a sphere
That's not the original question.

what would you see if you looked into it?
This clearly specifies that the observer is outside trying to see in. Since a perfect mirror would not let any light get out, nothing could be seen.

Defennder
Homework Helper
You're right Danger, I misread the question. Now that I've had a better look at it, I think you could visualise it as a 'greenhouse' effect where light could enter but not leave. Which is why nothing could be seen.

So, then, what would an inside observer see if someone pointed a flashlight at the sphere?

So, then, what would an inside observer see if someone pointed a flashlight at the sphere?
Umm. Someone pointing a flashlight at them superimposed over thier own internal reflection? Remember the sphere doesn't let light OUT. It doesn't stop it from coming IN.

The inside observer might also notice that it would be getting warmer. A perfect reflector would bend all that light energy back inside itself leaving the observer as the energy sink. Unless of course it was transparent to other radiation bands like infrared in which case energy buildup wouldn't happen. But that's outside the scope of the OP.

dst
I wonder if you had this sphere (and some ability to observe inside as well) and shined coherent, monochromatic light into it. I get the impression you would see alternating spheres of light like those russian dolls.

I just had an odd thought. Would not a perfect reflective internal surface make it impossible for light to get in in the first place? Here's my thinking. A mirror surface that reflects 100% of the light must be 100% covered by the reflective material. As opposed to say typical one way mirrors that are sparse coated to some degree. Since 100% of the surface is coated with material that is 100% reflective no light should enter or exit. From either side the light energy would be absorbed by the atom and re-emitted at the same frequency at 180 degrees to the incoming light. The outside would look like a perfect mirrored marble. The inside would be dark as sin until someone inside lit a match. And then he would go blind. But noone outside would notice. Except for perhaps the screaming in pain as his eyes suddenly get far more candlepower into them than expected.

dst
Well, it's a thought experiment. A perfect one-way mirror, spherical in shape.

D'Oh! Ignore my last post.

I went back and re-read the OP and a partially silvered mirror, such as the kind used in interrogation is specifically used as an example.

Given that bit of knowledge, if the lights were out inside the sphere and it was bright outside the sphere you could see out, but the outside could not peer in very well. (Barring acts that curtail the light such as cupping your hands on the surface and peering in) On the other hand if it were bright inside and dark outside it would be just like looking into a police interrogation room. If both sides were equally bright neither would see the other, just the reflection. If both sides were equally dim either could percieve the other, 'though darkly.

Claude Bile
Researchers have been using spherical micro-cavities with diameters of a few microns for a couple of decades now to obtain resonators with a high-Q that trap light for long periods of time (though note that the phrase "long periods of time" = milliseconds here). The light in these resonators are distributed in spherical modes termed "whispering gallery modes".

A cavity with perfectly reflecting surfaces (I use the term cavity because you don't necessarily need a sphere) would trap light indefinitely. As such, you could keep packing photons in there and the number would grow indefinitely.

It is worth noting that this is the practical application of High-Q resonators - pack lots of light into a small space and exploit the higher order nonlinear interactions that result.

Claude.

Danger
Gold Member
Would not a perfect reflective internal surface make it impossible for light to get in in the first place? Here's my thinking. A mirror surface that reflects 100% of the light must be 100% covered by the reflective material.
Yeah. That's what I was trying to get at in my first post about a 'perfect' 2-way mirror being impossible. It sure doesn't seem to me that there's any way to make one, unless there's some stuff that could be done with polarization.

edit: Sorry, Claude. I strolled down to the beer store in the middle of composing this, and thus didn't see your post. So how do you make it 2-way?

Last edited:
Claude Bile