This activity acts as an introduction to refraction. Prior knowledge: sound travels faster in solids (like steel) than it does in air. So what about light? Does it travel faster in transparent solids (like glass) than it does in air? Isaac Newton thought so.
We use the classic "car on carpet" model to explore the question. A toy car is rolled at an oblique angle down a ramp. There are hard surfaces, and there are carpeted surfaces. The direction of the car's path is affected by moving through the different surfaces.
After observing what happens to the car on a fast-slow-fast path and predicting what would happen on a slow-fast-slow surface, a ray of light is passed obliquely through a ray box prism (rectangular or trapezoidal). The path it follows reveals what happens to its speed while passing through the prism.
The lesson ends with this delightful refraction puzzle:
A ray of light enters from the upper left, heading toward an artistically shaped piece of glass as shown. The unrefracted path is shown. Sketch the actual (refracted) path the ray will follow.
I used an older version of Pasco's Basic Optics System. When you have a robust set of tools at your disposal, you can leverage those tools for grand experiments or wee demos.This is clearly a case of the latter. But it makes a case for having a tool set ready to go when the inspiration strikes.
Arbor Scientific's Light Box and Optical Set and Laser Ray Box and Lenses could also be used. I prefer white light to lasers in ray optics because white light rays can be dispersed into a spectrum, as we do in Diversion into Dispersion.
Other suppliers offer a variety of ray boxes and optical components, too. This Ray Box from Wards seems pretty groovy.
Student document (print-friendly Google Docs file on Google Drive)
Instructional presentation (link embedded in answer key)