Key Concepts
Light
Refraction
Reflection
Index of refraction

Introduction
If you pour water into a clear glass, what color is it? It's clear, right? But what happens if you try to look through it to see the world on the other side of the glass? It looks a little distorted, maybe a little fuzzier and uneven. If water is clear, why can't we see through it clearly? The answer has to do with how light moves through water, glass and other transparent materials. Similar to when you try to run in a swimming pool, when light tries to move through water or glass it gets slowed down. When light is slowed down, it either bounces off the material or is bent as it passes through. We can see these changes in light, which indicates to us that something is there. In this activity you will play with light to make normal objects appear and disappear!

Background
When light that is traveling through the air hits water, some of the light is reflected off the water. The rest of the light passes through the water but it bends (or refracts) as it enters the water. The same thing happens when light hits glass or any other transparent material. Some light is reflected off the object whereas the rest passes through and is refracted.

All materials have what is known as an index of refraction, which is linked to how fast light can travel through the material. As light passes through air and into another clear material (such as glass), it changes speed, and light is both reflected and refracted by the glass. This results in us seeing the glass because it reflects and refracts light differently than the air around it does. The change in the light allows us to differentiate one object from another. If a transparent object is surrounded by another material with the same index of refraction, however, the light will not change speed as it enters the object. As a result, you will not be able to see the object.

In this activity you will observe how the index of refraction of different materials helps us to see (or not see!) the objects as light passes through them!

Materials

  • Two clear glass jars, tall bowls or drinking glasses that hold at least eight ounces (Tip: Pyrex glass works especially well for this activity.)
  • Vegetable oil, approximately 14 ounces or enough to fill one of the glasses halfway (Tip: Avoid using “light” vegetable oil for this activity.)
  • Glass eyedropper (A plastic eyedropper or clear plastic drinking straw also will work. If you use a drinking straw instead of a dropper, each time you immerse the straw keep your finger over the top to avoid getting liquid in the straw. The instructions will tell you when to release your finger.)
  • Other transparent glass objects, such as marbles, beads, a magnifying glass or glass stirrers (optional)

Preparation

  • Fill half of one jar with the vegetable oil.
  • Fill half of the other jar with water.
  • Make sure your eyedropper is clean before starting the activity.
  • Set up a flat work surface that can be cleaned if any water or oil spills on it.

Procedure

  • Take your eyedropper (or straw) and, without squeezing it, immerse it in the jar of water. (For this step, avoid sucking up any water with the eyedropper or straw.) What do you notice about the eyedropper? Can you still see it? How clearly?
  • Keeping the eyedropper in the water, squeeze the top to suck up water. If you're using a straw, release your finger from the top to allow the immersed straw to fill with water. Did anything change about the eyedropper once it was filled with water? Does the eyedropper become easier or more difficult to see once it is filled with water?
  • Remove the eyedropper from the water and squeeze out all excess liquid.
  • Immerse the eyedropper in the oil, without squeezing it. Make sure to avoid sucking up any oil for this first step. What do you notice about the eyedropper? Are you still able to see it? Was it easier to see the eyedropper when it was in the water?
  • Squeeze the eyedropper to allow it to fill with oil. (If using a straw, remove your finger from the top to allow the immersed straw to fill with oil.) What happened? Can you still see the eyedropper? Is it easier or more difficult to see it now than it was when it was empty?
  • Remove the eyedropper from the oil in the jar and squeeze out the excess oil.
  • Slowly and gently pour the oil from the jar into the jar with the water. If you do this very carefully, the oil will sit right on top of the water! (It's ok if they mix though, they will separate once you stop pouring.)
  • Allow the oil and water to settle and separate (about one to two minutes). What do you notice about the oil? Are there bubbles in it? If there are bubbles, watch them closely and see if they are rising or sinking. If they sink, they are actually water bubbles trapped inside the oil!
  • Fill the eyedropper (or straw) with oil from the jar, and then slowly immerse it through the layer of oil, so that the dropper is visible in both the water layer at the bottom and the oil layer. Look at the dropper in the water layer, then in the oil layer. What is different about the dropper in these two layers? Is it easier to see the dropper in the oil or the water?
  • With the bottom tip of the dropper in the water layer, squeeze the dropper to expel the oil inside and allow it to fill with water. Again, observe the oil dropper in the water and oil layers. Is it easier to see the dropper in the oil or in the water this time?
  • Extra: Try repeating this activity using glass objects, such as marbles, beads, glasses or lenses. (Be sure you have permission to try out any object before using it.) Notice which things are the most difficult to see when you hold them in the oil versus the water. Why do you think that is?

Observations and results
Did the eyedropper become invisible (or at least harder to see) when it was full of oil and immersed in oil? This is what is expected. It may also have been hard to see when it was in the water (and full of water) as well.

The eyedropper “disappears” because of how we see light as it encounters glass. When light hits a glass object, some of the light bounces (or reflects) off the glass. The rest of the light keeps going through the glass object, but the light is bent (or refracted) as it moves from the air to the glass.

The index of refraction for the oil is very close to the index of refraction for glass. Therefore, as light travels through the oil and into the glass eyedropper, very little of it is reflected or refracted. As a result, we see only the "ghost" of the eyedropper in the oil.

More to explore
Refraction of Light Demonstration, from PBS Learning Media
Using a Laser to Measure the Speed of Light in Gelatin, from Science Buddies
Measuring Sugar Content of a Liquid with a Laser Pointer, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies