Science Questions

Can lasers be made to curve without a prism?

Sun, 19th Feb 2012

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Question

Glyn Edwards, Facebook asked:

Can you make a laser spiral or curve without the use of prisms?

Answer

Nic -  In free space, the laser has to travel in a straight line.  But as soon as you introduce curving in the medium - so imagine a laser going a liquid crystal cell which had changes in its refractive index as it went through - that would cause it to bend in the same way that a lens could make it bend.

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A cool demo (kitchen science-able, perhaps) is to mix sugar into an aquarium full of cold water and let it sit a while, then shine a laser through it.  You should see the laser bending downward.  This is because the denser, more sugary water settles to the bottom of the tank, which increases the refractive index there. There's a writeup with pictures on the Buffalo State website: http://physicsed.buffalostate.edu/pubs/StudentIndepStudy/EURP09/Sugar/sugar.html jpetruccelli, Wed, 22nd Feb 2012

Doesn't gravity do the bendy thing too, or is that because it bends space so that really the "laser" beam is still traveling in a straight line? Geezer, Wed, 22nd Feb 2012

It's defined as 'space' bending due to 'gravity', so the laser would still shine 'straight', although to you curve. yor_on, Wed, 22nd Feb 2012

It basically all comes down to the fact that light takes the shortest* path between two points.  If you have curved space-time, the shortest path is no longer a straight line, but a curved path (much like the shortest path along the earth's surface between two points is a curve).  If the light is traveling through some medium other than vacuum, that material slows the light down according to its index of refraction (higher index = slower).  If light travels through such a medium, it changes direction to minimize its travel time, which is why it bends.

There is a branch of modern optics called transformation optics which uses the mathematics of general relativity to design material structures that bend light in some desired way.  (The well-known optical "cloaks" are an example of this.)  This field works because light takes the shortest path in both the GR case and the case of a medium with a varying index of refraction.

*: Actually, light takes an extremal path, which is either longest or shortest.  jpetruccelli, Wed, 22nd Feb 2012

Many years ago there was research done on using hot air as a lens, with differential temperature being used to create a lens that was just hot air. SeanB, Thu, 23rd Feb 2012

Yor a fagit MeinKraftLover13370, Mon, 12th Oct 2015

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