In optics experiments, you often need to create lines of light. You can do this with light boxes, but they’re expensive, and tend to have too many rays to be useful. Laser light boxes are great, but again, spendy.
One teacher recommends using laser levels. These are the things made to help you hang pictures on a wall, so they’re level. Less than $20! It’s made of a bright laser, with a cylindrical lens, which spreads the dot into a line. You can see a demo here.
They create bright, dead straight, easy to use lines of light. They do seem useful for Snell’s Law, but it isn’t always obvious how to do it. The most common way is to use a semi-circular dish. You might find that your biology teacher has absonded with them, too. Filled with plain Jello, the rays are really clear. You can’t see the protractor I photocopied and placed under the tray, but it works great. You can also fill them with other substances like water and corn syrup to demonstate different indexes of refraction. However, ray needs something to scatter it so you need to stir in some milk or other colloid. Another option is to get a fine grit sand paper and roughen the bottom of the dish. This will scatter the light through diffuse reflection.
I also like to use plastic bars. You might already have glass plates, but they are surely too thin. The ray of light made by the laser level is thick and if the object is too thin, the ray will go over the top making for a confused appearence.
I made the plastic bar by buying a thick hunk of clear plastic from the discard bin at TAP Plastic. I had them cut it into pieces. I bought six grits of sand paper and progressively sanded the bars until they were clear. The final bit required a liquid polish. The people at TAP explained how to do it to me.
It’s pretty cool and can show total internal reflection really great.
Here’s a video from Teacher Tube:
Many thanks to Marc “Zeke” Kossover for this information.