Are there any clear objects that can actually "block white light"?

Question

some of my students are working with phosphorescent paper. They are looking for any object the "appears invisible" by being clear, yet still blocks the light? They want to put the object on the paper so that a person a few feet away can't see it, but when the phospor paper is hit by the light, the shadow of the object apears.

I think it's a very cool idea, but I also a think it's a real long shot. I really need some help from you science geniuses that hang out here.

Answer 1

This is one of the toughest questions I've come to try to make sense out of this, but I'll try: Imagine a magic glass ball in front of the phosphor paper which appears invisible because it's clear magic glass. Yet, when a lamp is turned on, it casts a shadow on the phosphor paper. Why it has to be phosphor paper and not an ordinary one I have absolutely no idea, but I'm going along here. I think what you want to do is to turn on the lamp, wait a while, turn it off, and then remove the magic glass ball, revealing a persistent "shadow" on the paper, because supposedly no light went through to excite it. All right. What you are saying is that 1) the magic glass ball looks invisible because it's so clear that light FROM the phosphor paper is coming through without any impedence, and yet 2) the magic glass ball BLOCKS the lamplight from reaching the phosphor paper! A tall order! That's why it's called a MAGIC glass ball. The military is now studying metamaterials that have interesting cloaking properties, theoretically rendering things "invsisible", but it's invisible in BOTH directions. But obviously if the the phosphor paper "cannot see" the lamp, then the ball isn't so invisible from the phosphor paper's point of view, right? But in science, we always try to do the impossible. How can we do a such a thing? Hmm. ...

An older "invisibility" technology also being investigated by the military is the "chameleon" technology, which involves the use of flexible display screens. A solder wearing this special suit would "pick up" the color and patterns of the field behind him, and through circuity, project it on the fabric screen in front of him, so that a person standing some distance away will only see the scene behind him. Since this type of technology allows control over image projection, it can arranged so that light and patterns appear to go one way, while barring light going the other, thereby helping create the shadow on the phophor paper.

In optics, it's a fundamental fact that electromagnetic waves travelling through media is completely reversible. Kind of like signs on semi trucks saying, "If you can't see me, I can't see you!" However, maybe it's possible with non-linear optics where a material could actually have the property where dim light is passed, but bright light is partially blocked, so that that would be the closest thing to having that magic glass ball. It would appear to be fairly clear in front of of the phosphor paper, and yet block SOME of the bright light from the lamp, so that it would leave a "shadow" in the phosphor paper.

Here's a link (last one) to an abstract about such a thing, "Nonlinear absorption phenomenon in optical materials for the UV-spectral range" The concept is there.

Answer 2

If you are using fluorescent paper, ordinary glass would block enough of the ultraviolet light (that is the source of the most of the fluorescence) to leave a shadow. Thin glass on a plastic film would be difficult to see in white incandescent light and the difference in transmission of ultraviolet-rich white fluorescent light would show a shadow.
The shadow is not the light from the paper being blocked but the light going to the paper.

Answer 3

By definition, if an object is clear, it does not block light. White light is merely the combination of all colors of light.

Very recently, there have been some developments that suggest that a 'cloaking' device that makes objects invisible might not be too far off, but it's still mostly speculation and only in labs.

http://www.eetimes.com/news/latest/showArticle.jhtml;jsessionid=OL3DLBKQK0QCQQSNDLSCKHA?articleID=193400671

Answer 4

Hi. This is off the wall but a magnifying glass blocks light anywhere but in the refraction cone. Maybe a refractive piece of glass?

Answer 5

I don't know if this is what you're looking for, but you could try a pair of polarizers. They're transparent when oriented a certain way, but If you turn one of them 90 degrees they'll block out any light.

That's the best I can come up with.

Answer 6

If you can see through it, that means light goes through it - therefore it doesn't block white light. As I'm sure you know, white light consists of all the colors of the rainbow.