Enlighten Your Day with Optics!

This activity uses lenses and prisms to explore optical reflection and refraction. The initial activities take 30-35 minutes – followed by a lot of free play! Intended for ages 5 and up.

What you'll need:

1. An assorted set of cylindrical lenses and prisms (PH0576)
2. Computer paper and scotch-tape.
3. A cell-phone with an LED flash that can be operated like a flash-light.
4. Some duct tape is helpful but not necessary.

Getting Set-Up:

The first thing to do is set up an optics workbench. This is as easy as taping some computer paper to a table close to the edge of the table (this part is important). We suggest taping two pieces with long sides touching so the area is 17” wide and 11” inches tall. You can add more paper later if necessary.

The reason to set up the optics workbench close to the edge of the table is so that the light source can be mounted so the light comes out of the source just above the table surface. The best types of light sources have a single LED that emits in all directions. Cell-phones typically have an LED on the back which is mostly used with the camera but can also be used as a flash-light.

You can either hold the cell phone when you need the light, or you can use the duct-tape to build a little caddy or shelf which hangs off the end of the table. Either is fine.

Now that you’re all set up, the best place to start is with the semi-circle lens. Place it on the table with the flat side toward the light source. See if you can adjust the position of the lens to make a collimated beam of light. A collimated beam travels in one direction and doesn’t spread out. Once you feel good about the beam, we suggest you tape down the lens using the Scotch tape so you don’t have to readjust it. To make a better beam, you can tape some paper to the edges of the flat surface so the light only travels through the central 1-inch region of the lens.

Time to explore!

1. Take the rectangular prism and put it in the beam path. What happens to the beam as you slowly rotate the prism? (You should see the beam move side to side, depending on the angle of the prism). 

What you are seeing is optical refraction. When light goes from free space into a transparent medium at an angle, it bends. When it comes out again on the other side, it bends in the opposite direction, undoing the first bend. This results in a translation of the beam. The steeper the angle the larger the bend, so the greater the translation.

2. Is there any translation if the rectangle is parallel to the beam?

The same type of bending happens in all transparent medium – water, glass, plastic, etc. The shape of the transparent medium is what makes a lens work because different parts of the beam are bent a different amount.

3. Remove the rectangular prism and put the convex lens (it looks like the top view of a canoe) in the beam path. What happens to the beam now?

The round shape of the lens surface sends the beam into a focused spot.

4. Remove the convex lens and put the concave lens (it looks like the letter I) in the beam.

The inwardly curving shape of the lens causes the beam to diverge.

5. Return to the convex lens. What happens to the light after the focused spot?

The light diverges after the focused spot.

6. Try to put the concave lens in the beam after the convex lens. Adjust the position of the concave lens until the light coming out the far side is collimated again.

Though the beam was converging toward a focused spot, the convex lens made the light spread out again into a collimated beam. This is all because of the shape of lenses.

7. Try reversing the order of the lenses, putting the concave lens first followed by the convex lens. Are you able to make the beam collimated again? Is the beam bigger or smaller in this case versus the previous case (step 6)?

8. Finally, remove the concave and convex lenses and put the triangular prism in the original beam path with the light entering one-side only, but at an angle. You should see a rainbow of colors after the prism.

This happens because optical refraction is slightly different for each color of light. Different colors bend differently. In a rectangular prism, the different colors travel at slightly different angles but they’re mostly put back together on the other side. But in a triangular prism, the colors are further separated at each surface so they come out heading in different directions. As the beam travels away from the triangular prism, the different colors separate making a very nice rainbow.

9. Experiment with changing the width of the beam before the triangular prism. (You can use your fingers to block parts of the beam. How does this change the character of the rainbow?

The triangular prism shows that light is made up of different colors. When we see white light, we are seeing a combination of many different colors.

10. What else can you do with prisms and lenses?