MadSci Network: Physics

Re: Are there materials which allow light to pass in only one direction?

Date: Wed Apr 7 20:17:53 2004
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 1080741076.Ph

Are there materials which allow light to pass through them in only one direction? Hmmm... If I understand you correctly, you mean, for example, a window which allows light to go from the indoors to the outdoors, but not from the outdoors to the indoors. In other words, a one-way window.

You may have heard of "one-way mirrors", which appear to meet your needs. A person in an interview room, for example, may see what looks like a mirror on the wall. He might look into it to comb his hair. But on the other side of the wall, researchers can see exactly what he is doing. This sounds like a "one-way" material, but it is really just a trick.

As you can read on this site,

a "one-way" mirror is really just a piece of glass with a thin silver coating. A small fraction -- say, 10 percent -- of the light passes through the silver coating in each direction. However, if you mount this piece of glass in a hole between two rooms, and light one room brightly, while leaving the other room dark, then it acts like a "one-way" device. There's lots of light in the bright room, so even though only 10 percent of the light rays go through the coating, that's a lot: a person standing on the other side of the glass (in the dark room) can see what's happening in the bright room very clearly. On the other hand, since there are very few light rays in the dark room, and only 10 percent of those go through the coating, very few photons leave the dark room to fly into the bright room. The unsuspecting interviewer will notice only the (partial) reflection of his face, without realizing that there is a very, very dim view of objects in the other room, on the far side of the glass.

However, if we lit both rooms equally brightly, a person looking at the glass would notice something funny about the reflection; and, if he investigated closely, would be able to see objects and people in the other room.

So this partially-silvered mirror glass isn't REALLY a one-way optical device; instead, it's just a trick in which lots of light is sent one way through the glass, and very little is sent the other way.

Could you find a REAL one-way device? Unfortunately, no. The laws of physics -- specifically, the laws of thermodynamics -- indicate that it is impossible. Let me give you an idea for the reason we believe this is true. Suppose that there was such a material. Well, then, I could take six pieces of it and make a simple cubical box, with the material arranged so that light could only go INTO the box, but not come back out. Suppose I then placed a cup of water into the box and put on the lid, so that it was completely closed. What would happen?

Some light from the outside would go into the box and hit the water, warming it up slightly. Under ordinary circumstances, the warm water would radiate away energy and cool back down. But, if there really were a one-way material, it would prevent any of the radiation from escaping from the box; it could only bounce around inside. That means the water would re-absorb it and stay warm. As more light from the outside world entered the box, the water would absorb more and more energy, becoming hotter and hotter. It could never cool off, and would eventually (in theory) reach an infinite temperature! That can't happen in a real universe.

I wonder if your idea for an energy source was built around this principle? If so, you are very ingenious ... but it's not going to work :-/

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