Index of Refraction

Activity/Demo overview:  This is a short demonstration of how indices of refraction work. Because vegetable oil and borosilicate glass have the same index of refraction, glass test tubes disappear when they are submersed in the oil. This demo is very short, and if you present it like a magic trick, this works very well at the start of a class or presentation to set a fun tone with students and warm them up to the idea that physics is fun and interesting.    Activity/Demo Instructor Notes:  Materials:

• 1 beaker large enough to fit multiple test tubes
• Vegetable oil
• 3 borosilicate (Pyrex) glass test tubes (1 broken, 2 not)
• Tongs
• Paper towels

Preparation: Put the whole test tube in the beaker, and fill the beaker with enough vegetable oil to completely submerge and fill the test tube, but leave enough room to add another test tube and stir. The test tube should be hard to see in the beaker, particularly from a distance.  Presentation:  When you start the demo, you can introduce it as a magic trick. Show the students the broken test tube that you haven’t put in the oil yet. Then put the parts of the broken test tube into the oil with the tongs, making sure no air is left in any of the pieces so that they completely disappear. Stir the beaker with the tongs, then pull out the whole test tube as if you’ve just fixed the broken one.  After pulling out the “fixed” test tube, ask the students what they think just happened. Some students might already know the answer, so you may want to ask those students to hold off on telling the class the correct answer so other students can try to figure out what’s going on. This helps students get comfortable with sharing ideas that may be wrong and can help you start building rapport with the class. Discuss the students’ ideas with them for a couple minutes, helping them to see why their answers may not make sense and guiding them toward the correct answer without giving it away.  If any students know the answer, you can let one of them share it after discussing others’ ideas. Then you can provide a more detailed explanation that materials have indices of refraction that describe how much light bends when traveling through the material, or how much the speed and wavelength of the light changes. Pyrex glass and vegetable oil have the same index of refraction, so light doesn’t bend when it moves between the test tubes and the oil, making them appear to be made out of the same material. You can emphasize this by pulling the broken pieces of the test tube out, showing that you didn’t actually fix the test tube, you just had a whole one in the oil already that they couldn’t see. You can then put the whole test tube back in without letting it fill with oil, and explain that air has a different index of refraction, so you can see the test tube since the light bends as it moves between the glass and the air. You can also connect this to prisms or rainbows, and explain that in some materials (water, for example) the index of refraction varies with wavelength, which splits the beam of light and produces a rainbow. 

Alysa Rogers, 2022-23 Wonders of Physics Outreach Fellow