Invisible Glass

SKILL LEVEL

K-6, Middle school, High School

TRY IT!

Have you ever been in a swimming pool, looked down at your feet, and noticed that they did not appear to be where they were supposed to be?  Or maybe you've tried to dive for a toy or a coin underwater, only to find that it wasn't where you saw it?

In both cases, the light reflecting off of the underwater objects is bending as it enters the air.  This phenomenon is called refraction and it occurs whenever light speeds up or slows down as a result of entering a new medium.

The amount of refraction can be quantified with the refractive index.  The refractive index of a vacuum, such as in deep space, is exactly 1.  Any matter that would slow down the speed of light will have a refractive index greater than 1.  Water, for example, has a refractive index of 1.33.  Pyrex glass has a refractive index of 1.47. The refractive index of air is just very slightly greater than 1 (1.00029).

Have you ever noticed your faint reflection in a pool of water or in a glass window?  Whenever light passes between two substances with different refractive indices, reflection can also occur (thus, you can see your image in a placid pool of water).

In this experiment, you'll use two substances with nearly identical refractive indices (vegetable oil and glass) to eliminate reflections and make the glass appear to become invisible!

SUPPLIES, TOOLS, MATERIALS

  • Two glass beakers or cups, one small enough to fit easily in the other
  • Vegetable oil, enough to fill the larger container.  We used Wesson brand.
invisible glass

INSTRUCTIONS

  1. Wash both containers with soapy water.  Rinse thoroughly, making sure there is absolutely no soap residue remaining and both containers are crystal clear.  Dry both containers completely.
  2. Fill the larger container will vegetable oil.
  3. Slowly submerge the smaller beaker into the larger one.
  4. Record your observations!
invisible glass 2
invisible glass 1

QUESTIONS TO CONSIDER

  1. Look up and define the following terms:
    Reflection
    Refraction
    Refractive index
  2. Record your observations when you submerged and removed the smaller beaker from the larger one.  Explain what was happening.
  3. The refractive index of water is 1.33.  If you used water instead of vegetable oil, would the beaker have become invisible? If not, why not?  Try it and record your observations!