Okay - its not a CLOAK, per se, but it is a cloaking technology - and its a super simple set up to see how one of the most promising approaches to invisibility technology that we can use (think surgeons being able to see through their hands during surgery). This is a replica of the Rochester Cloak - developed last year at the University of Rochester in NY.

Invisibility happens when an object that “should” bounce light into your eye does not. The object is present, but no light bounces from it into your eye, so it appears to “not be there”. Now, you can simply cover an object and no light from it will get into your eye, but that is not really invisibility in the way that we like to think about it. What really mean is that we are able to see around and behind the invisible object, as though it were not there at all. That’s a little more tricky.

One way to achieve invisibility is to bend light (refraction) around an object -- one way to do that is with lenses.

This project uses 4 simple lenses, a meter stick and some very simple math to create regions of invisibility.

## Step 1: Gather Your Materials

To create your Invisibility Cloak, you will need

4 Double Convex Lenses; 2 of one focal length and 2 of another. Any two focal lengths will work - I purchased a KIT that has 2 lenses with focal length 50mm and 2 lenses with focal length 150mm. You can read more about lenses and focal length HERE.

Some kind of STAND to hold your lenses. Again, I bought a KIT so you can see mine in the picture here.

A meter stick

A long, flat surface - a table is better than the floor since it will be easier to see down the axis of the lenses

Some Math...very simple math... don't freak out!!!

## Step 2: Setting It Up (and the Math)

The trick is setting up the lenses at specific distances from each other along an axis (meter stick)

By setting up the four lenses as shown you can create cloaked regions. Each “set” is composed of two lenses (f1 and f2 ) at a distance from one another (d1). Then each “set” is set at a distance from the other (d2).

The distances between the lenses and the sets of lenses depends on the focal lengths of the lenses you start with and is defined by some simple math:

Start with 2 sets of lenses with different focal lengths. For the lenses i have from the KIT I bought, the focal lengths are 50mm and 150mm - this makes it so the entire set up fits neatly on a single meter stick. But really - any two positive focal lengths will do.

Separate F1 from F2 by the sum of their focal lengths to arrive at d1, the distance between the two lenses in each set . In this case, with my lenses from the kit:

d1 = (F1)150mm + (F2) 50mm = 200mm

Separate the two sets according to the formula* so that the two f2 lenses are d2 apart.

*d2= 2 f2 (f1 + f2) / (f1— f2)

d2= 2x50mm(150mm+50mm)/150mm-50mm

d2= 100mm(200mm)/100mm = 200mm

So...in this particular case, both d1 and d2 = 200mm (or 20cm)

## Step 3: "Seeing" the Invisibility

Now for the fun part. That is me demonstrating this at a conference.

It is also easier to see if you have some kind of patterned or brightly colored background or wall at the end of the set up.

As you look down the axis of the lenses, have someone slowly move something between the lenses 1 and 2 and then again between 2 and 3 and finally between lenses 3 and 4.

There are areas of invisibility in each of these areas and they are not all the same -- some are bigger than others and some are at the BOTTOM of the lenses. Experiment. Have fun.

If you are so inclined, there is more to read about the math and physics of what's going on, published by folks who created the simple set up for the Rochester Cloak.

Cool : o
<p>Great optical trick.</p>