Hi Jon --
Nice try. Unfortunately, the path light takes is not affected by the presence of a magnetic field. Light itself is composed of an oscillating electric and magnetic field, and one very important property of electric and magnetic fields is what we call "linearity." That is, if you have two sources of electric and/or magnetic fields, you can predict what the combined field is just by adding the two source fields together. The two fields donít change each other at all. So if you add the field of a light ray to any other field we can imagine, the light ray will continue as before and the extra field will just stay the same, adding to it in places where the extra field is strong, but having no effect beyond the reach of the extra field. So there is no way that a magnetic field can bend light.
Although magnetic fields might not do the trick for you, there is quite a bit more about light that can be taken advantage of. For instance, if the object is very small (small compared to the wavelength of the light), the light will simply diffract around the object and be invisible or nearly so anyway. Please see our answer about diffraction
Also, even though a magnetic field wonít do anything for the light, a gravitational field, sufficiently strong, will in fact bend light. This was observed early in the 20th century confirming Einsteinís General Theory of Relativity in which light from the planet Mercury was bent by a very tiny amount by the enormous gravitational field of the sun. Unfortunately they needed a total solar eclipse to block out the bright sunís rays so they could see the feeble light from Mercury on the other side of the sun passing close by where the effect was big enough to be measured.
One of the more spectacular demonstrations of bending light by a gravitational field is "gravitational lensing" of light from very faraway bright objects whose light passes close by another distant object with plenty of mass. Here is a description, along with some *very* nice pictures: http://imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html
The problem with making an object invisible in this way is that the light cannot be bent any way desired -- what you get is just lensing, and you get multiple images of the object in back of the object doing the light-bending. An object like this will attract quite a lot of attention! These galaxies sure attracted our attention. And you need a galaxy-sized gravitational field to do the trick.
To make a gravitational "invisibility cloak" requires quite a strong gravitational field. Maybe the best way to do it is to have such a strong field that the light just gets sucked into the object and cannot come out. Then you have a black hole. Not invisible because you can tell that your light is gone, but interesting nonetheless. Hereís a tutorial on black holes: http://www.damtp.cam.ac.uk/user/gr/public/bh_intro.html
Now the disclaimers back on your original question: If your magnetic field is strong enough over a large enough distance, you can have enough energy stored in it to do gravitational lensing, and then refer to the above answer on gravitational lensing. This however is a very difficult way of getting a strong gravitational field. It is much easier just to collect a galaxyís worth of matter than to collect the equivalent energy in a magnetic field (neither is particularly easy, I admit!)
The second disclaimer is that there is a small expected deviation from linearity of electric and magnetic fields due to quantum mechanics and the ability of electrons to pop out and go away on microscopic time scales. This only becomes noticeable for very very high frequency light colliding with other very very high-frequency light (it wouldnít be noticeable and may even have exactly zero effect for a static magnetic field and visible light -- I havenít done any calculations). There are plans to make such a light-light collider, but it requires a many-mile electron accelerator to get the energy of the light high enough.
If you are really interested in getting light to go around a solid object so that it looks like it went through, the low-tech solution might be the easiest. Magicians use mirrors for this purpose all the time!
(republished on 07/29/06)