Progress Toward an Invisibility Cloak?

By Betty Robinson,

With starship cloaking technology and Harry Potter’s invisibility cloak, the ability to hide in plain sight has been fiction. But different technologies are being developed to turn science fiction into reality. Some of the reasons why we would want to do this are just because it’s fun and for military advantages. There are some potential medical applications—for instance, getting the right cloaking device could allow surgeons to see through their hands and work on something hard to reach or see. And no doubt there will be uses no one has thought of yet.

One of the technologies is at the nanoscale and involves materials science. Another demonstrates the effect with optical lenses. Whatever the technology, it comes down to light. If light reflected from an object doesn’t reach our eyes, then we can’t see the object.

The cloaking device I’m demonstrating here is called the Rochester Cloak, developed at the University of Rochester by John Howell and Joseph Choi. The great guidelines that I used can be found here: This device uses lenses to manipulate light so that it does not interact with the object to be cloaked—light doesn’t hit the object, light can’t reflect off the object to our eyes, so we can’t see it. But we can still see the background.

I just set up the device according to the instructions in the above pdf; there is even a suggested shopping list and suggested retailer (; an American company, but they ship to Canada). The focal lengths of the lenses are key to the Rochester Cloak, so you have to know their values—the focal lengths of the first two lenses must meet, and the same for the other two lenses. And you have to know the focal lengths to work out the distances between the lenses. But the pdf above spells it all out very clearly.

You set up four convex lenses, with two different focal lengths, according to a formula provided in the pdf. The lenses with the shorter focal length (thicker lenses) are in the middle; the lenses with the longer focal length (thinner lenses) are on the ends. And you put a background of some kind at the end of the setup. It’s good to use something with a grid, like graph paper, so that you can see there is no distortion of the background.

Pockets, or cloaked regions, are formed where the light coming through the lens doesn’t interact with anything placed in it. In the illustration below, these are the areas with red squiggles. So if you place an object in the cloaked region, the light doesn’t interact with it, and we just see the background. No interaction of light with the object, no light reaching our eyes, no object to see.

The blue lines are rays of light refracting through the lenses. The rays cross where the two focal points meet. The areas with the red squiggles are the cloaked areas. The light coming through the lenses does not pass through these areas so we can’t see an object placed in the cloaked areas. The cloaked areas are above and below the centre line. Objects placed in the centre line do not get cloaked.