Index of Refraction

Activity Overview: This demonstration very clearly shows the impact of the index of refraction on the path of light as it crosses through various mediums. The materials are flexible and fit on a tabletop. Activity Instructor Notes: Materials: 
  • Fish tank, pyrex dish, or other square glass dish
  • Laser pointer
  • Wide aperture flashlight
  • Dark construction paper
  • tape
  • Smoke of some kind (liquid nitrogen, hairspray, etc. You just need to see the laser path through air
  • 8 quarters/washers
  • Water source
  • 1-2fl oz of milk
  • Optional: protractor
  Preparation: You need to make a mask for the flashlight. Cut the paper in half. Cut a semicircle out of each half and tape them over the aperture of the flashlight so that the flat side of the two semicircles face each other. Leave enough room so that a long slit of light passes through unobstructed. Next, place the glass container on top of the quarters/washers so light from the flashlight passes through the glass and underneath. Fill the glass with water and very incrementally dilute the milk into the water. The purpose of the milk is to increase visibility of the light from the flashlight as it passes through the water. Add too little and it will be difficult to see. Add too much, and the light will scatter too quickly to see at all.    Presentation:  Start with a quick introduction of the topic: index of refraction. You can do this by asking if the audience has ever noticed objects look distorted when observed underwater. Explain this is based on how the light’s speed changes in materials like water, and that this effect is measurable.   Draw attention to the water dish. Explain that water has an index of refraction of 1.33 and that air’s index is 1. Point out that the glass is arranged such that the audience will see light pass through the water and underneath through air only. Turn on the flashlight and set it on the table pointed towards the dish. Slowly rotate it such that light is incident on the glass at varying angles. Point out that the light beams are aligned when you point the flashlight straight at the glass and diverge from each other as you increase the angle of attack.  If the dish is tall enough, repeat from above with the laser pointer. Start by releasing a small amount of smoke (the easiest is probably nitrogen cloud vapor from a liquid nitrogen dewar) onto the surface of the water. Point the laser at about 45 degrees relative to the surface of the water. Point out that the angle of the beam bends at the surface of the water as it enters the water, but that part of the beam reflects away from the surface at the same angle as the incident beam.   An optional element to this demo if the audience is small is to allow members of the audience to measure the angles using a protractor.

Gavin Chase, 2023-24 Wonders of Physics Outreach Fellow