Build a Physics Apparatus: Ideas Needed

[physux212]

my fourth quarter project is that i have to build a physics apparatus. it can be a model but i think my teacher prefers it if the apparatus i build actually performs the task it's supposed to. I've been researching but i can't find any good ideas. building a circuit is an obvious choice but its too easy. does anyone have any ideas of an apparatus that is relatively easy to make and will work? any ideas will be greatly appreciated.

 

[Danger]

How about grabbing a $2 laser pointer and building an interferometer?

 

[berkeman]

-- Beam balance (how accurate can you make it?)

-- Use stepper motors to make an x-y table

-- Microscope (and quantify the aberrations...how many elements should you use?)