Total Internal Reflection

When light hits an interface between two different media, it can behave in two different ways. Typically, the light partially refracts (bends) and partially reflects. The refractive index of the material is what determines how fast light travels in a material. Light travels about one and a third times faster in air than in water. Learn about total internal reflection in this activity.

Materials

  • Laser pointer (preferably green)
  • Empty 2-liter soda bottle (clear)
  • Rubber plug
  • Bucket
  • Water

Directions

 
  1. Make a circular hole close to the bottom of the empty soda bottle. The hole should be about 1 cm in diameter; big enough so that the stream is clear, small enough so you can observe the guided light for long enough to see the effect. Plug the hole with the rubber plug or your thumb and fill with water.
  2. Situate the soda bottle above the bucket. Try this a few times so the area stays dry. You might want to have an assistant hold the bottle.
  3. Aim the laser pointer through the soda bottle to the hole. As a safety precaution, do not point the laser at anyone!! The laser light can reflect and refract in unintended directions, so practice before inviting friends to watch. If you do have help, your assistant could block stray light with a card and have only the hole visible to your friends.
  4. Remove the plug and allow the water to pour into the bucket.
  5. You should be able to see a green splash of light where the water lands in the bucket and some of the reflections of the green light in the stream of the water. The light is being guided through the water, which has a higher refractive index than air. You can also experiment with different clear materials such as glass.
  6. Turn off the laser before the water level gets to the hole.
  7. One application that makes use of total internal reflection is optical fiber. Typical optical fiber is made from glass and there is a core and a cladding. The core is where the light is guided and has a slightly higher refractive index than the cladding. Optical fibers are used for many applications such as telecommunication, sensors, imaging, and illumination.
 

Explanation

When light hits an interface between two different media, it can behave in two different ways. Typically, the light partially refracts (bends) and partially reflects. There is a special phenomenon that occurs when light goes from a higher refractive index medium to a lower refractive index medium and above a certain incident angle. The refractive index of the material is what determines how fast light travels in a material.

We can take the interface between water and air as an example. Water has a higher refractive index than air. When light is propagating from water to air, for all angles less than a certain critical angle, the light gets transmitted through. But once the angle is larger than the critical angle; all of the light gets reflected. This is what is called total internal reflection. This is explained by what is known as Snell’s law, which shows the relationship between the incidence angle and the refractive index of the material. The equation for Snell’s law is ni sin ϴi = nt sin ϴt, where n is the refractive index of the material, ϴi is incident angle, and ϴt is transmitted angle. So for the critical angle where there is no transmission, ϴt = 90° and Snell’s law can be rewritten such that

 

ϴi = sin -1 (nt / nt).


In this case, nt is the refractive index of air and ni is the refractive index of water. The refractive index of air is 1.0 and the refractive index of water is 1.33 so the critical angle for total internal reflection for a water-air interface is 48.7°.

 

Total Internal Reflection 01