While many students would surely like to own a 48-inch multi-touch screen, most don’t have the cash to buy a Microsoft Surface device. But a group of engineering students at the University of Waterloo in Ontario didn’t see that as a problem: they just built their own.

Their device does not strictly work on touch, but rather light. It’s based on the concept of total internal reflection, which the students kindly describe as “grade 12 physics”. For those of us who didn’t quite master that level, it’s the optical effect of the way different surfaces have different refractive indexes (that is, they reduce the speed of light at different rates.)

If you’re underwater, for example, and look up, you’ll see a mirrored reflection on the “underside” of the surface, instead of seeing through to the open air. Light is reflected back rather than passing through to the air above.

In the specific case of the touchscreen, the students took advantage of this phenomenon by layering a sheet of Plexiglass (which traps the light like the surface of water does) and another material with a higher refractive index to create a sort of “surface.” Pressing down on the screen disrupts the refraction and allows light to pass through. In our swimming example, if you float a rubber ducky on the surface, the underwater swimmer will be able to see the rubber ducky “pass through” the mirror seen on the surface.

With the students’ touschscreen, the fingertip disrupts the light at the surface. This is then detected by a camera which takes an image and then works out where on the screen the disruption took place, and thus where you are touching. Set the camera to repeat this process fast enough and you have an operative touchscreen with very little logistical limits on its size.

(One last thing: Normally we wouldn’t mention the demographics or background of a scientist or engineer unless it was strictly relevant: geekdom is an equal opportunity awesomeness. But in this case, while gender really isn’t the story, it’s kind of cool to note that the student posting the report did so on an all-female engineer students blog, which specifically has goals including “tackling the stereotype of engineering as a male-dominated profession.”)