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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:
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!
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.
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.
The round shape of the lens surface sends the beam into a focused spot.
The inwardly curving shape of the lens causes the beam to diverge.
The light diverges after the focused spot.
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.
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.
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.