An enlightening activity from Science Buddies

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!

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

Patrick Jackson and The Conversation US

William Gallus and The Conversation US

Andres Picon and E&E News

Fionna M. D. Samuels and Tulika Bose

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