# How To Make An 'Invisibility Cloak' At Home For Less Than \$100

Jessica Orwig

Physicists at the University of Rochester have created an incredibly versatile cloaking device, which hides things from view. But more importantly, you can make this crazy concealing device at home for under \$100.

All it takes is four lenses, an optics bench for holding the lenses in place, and an insatiable appetite for making things disappear.

The four lenses, when aligned just right, will bend light around the object you place between them, cloaking the object in the process. Watch this device, called the Rochester cloak, in action below, "cloaking" the researchers hand:

Lenses, like the convex lens below, bend rays of light. When a lens is convex, these bent light rays focus to a single point.

The distance from the center of the lens to the focal point is called the focal length, which is shown below. How strongly a convex lens can magnify an image depends on its focal length. The shorter the focal length, the stronger the magnifying power of the lens.

What you will need to do:

1. Obtain two sets of two lenses with different focal lengths. The first set will have one focal length while the other set will have a different focal length. You will have four lenses in total, which should cost you no more than \$30. The lens provider will include the focal length information (sometimes denoted as FL) so you don't have to calculate it yourself.
2. Using an optics bench, select one lens with the first focal length and a second lens with the second focal length. Separate them by a distance that is the sum of their focal lengths. For example, if your first lens has a focal length of 5 inches and your second lens has a focal length of 3 inches, then separate these lenses by 5+3 = 8 inches.
3. Now, do the same with your remaining two lenses.
4. Lastly, you need to know how far apart to separate your two sets. This will take a little math, but here's an example using the same measurements in Step 2: D=[2 (3) (5+ 3) ]/ (5— 3) = 12 inches should be the distance between your two lenses with the focal length of 3 inches.

You can see the full equations with a diagram on the University of Rochester's website. Here's a helpful diagram to give you a better idea:

They used lasers to show how each of the four lenses bends light rays to recreate the image at the back of the set up, even if something's in the way between lens 1 and 2. You can see a great example of the laser rays converging at the focal point after exiting lens 1 located at the far left.

The device does have its limits, though. The device only cloaks at 15 degrees in either direction from where you deviate your eye from the central axis of the lens. But that's 15 degrees more than any former cloaking device. Here's the video, from Rochester University, with more details: