May 2016

Light Refraction (5/4/16)

MATERIALS:

  • Large clear glass or plastic containers
  • Pyrex glassware (Lab beakers, test tubes, and some kitchen glassware are made from Pyrex glass. Just be sure it is Pyrex!)
  • Vegetable oil (Any brand of oil will work, but Wesson brand oil has less of a yellow color.)
  • Clean tap water
  • Rubber gloves

INSTRUCTIONS:

  1. Put on your rubber gloves (unless you want to have super slippery hands!)
  2. Fill a Pyrex beaker with vegetable oil. Be careful not to spill any of the oil, as it can make a very greasy mess!
  3. Slowly lower the oil-filled Pyrex beaker into a glass full of clean tap water. Can you see the Pyrex beaker through the water? Can you see the oil inside of the Pyrex?
  4. Now slowly lower the oil-filled Pyrex beaker into a glass full of pure vegetable oil. Can you see the Pyrex beaker now? Can you see the oil inside of the Pyrex?

YOU CAN ALSO TRY:

  1. Fill a glass about halfway with clear water, and halfway with oil. Allow the oil to rise to the top of the water before slowly lowering the oil-filled Pyrex beaker into the glass. What did you observe this time?
  2. Repeat the experiment with a non-Pyrex glass beaker. Will it still disappear in the oil?
  3. Fill the Pyrex beaker with clean water and repeat the experiment. Will you get the same results as before? Why not?
  4. If your container is tall enough to hold the entire Pyrex beaker inside of it, why not try to submerge an empty Pyrex beaker into the oil? Slowly lower the empty Pyrex beaker into the vegetable oil until the oil begins to pour in over the top of the beaker. What did you observe as the beaker filled up with oil?

HOW DOES IT WORK?

An object is only visible when it can reflect or refract light. Transparent objects—like Pyrex glass—do not reflect (or bounce back) much light, but they do refract (or bend) light coming from behind them.

When light passes from one substance into another substance, its speed and direction can change slightly. The amount of change in direction of light is measured by the Refractive Indexes of the two substances. If they have very different Indexes of Refraction, then the light will bend a lot as it passes from one substance into the other. If the two substances have similar or the same Indexes of Refraction, then the light will bend less or not at all.

Index of Refraction

Water – 1.333

Vegetable Oil – 1.471

Pyrex – 1.474

Glass – 1.458

When you put the oil-filled Pyrex beaker into the water, it remains visible because water has a much different Refractive Index than the Pyrex glass. Light is bent when it passes from the water into the Pyrex glass, and again when it exits the Pyrex back into the water. This refraction allows us to clearly see the beaker inside of the water.

Things start to get a little more interesting when you lower the oil-filled Pyrex beaker into a glass full of vegetable oil. Because the Pyrex and the vegetable oil have almost the same Indexes of Refraction, light can travel between the oil and the Pyrex in a nearly straight line. The clear Pyrex becomes almost invisible within the oil because the light passes right through it!

Now go and show everyone your new Super Science Power!