Index of Refraction – Ruler Changes Size Science Experiment

Would your eyes fool you? Don’t be too quick to blame them! But if it’s not our eyes, then why does the ruler look wider in this science experiment?

Print our materials list and experiment instructions, preview our demonstration video, then learn how this experiment works with our easy to understand lesson on refraction.

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Supplies Needed

• Tall empty clear glass
• Water
• Cooking Oil
• Rubbing Alcohol
• Ruler

Ruler Changes Size Science Experiment Instructions

Step 1 – Pour water into a tall clear glass. Fill it about 1/3 of the way full.

Step 2 – Next, carefully pour in cooking oil into the glass until the glass is a little less than 2/3 of the way full.

Step 3 – Slowly pour rubbing alcohol into the glass until the glass is almost full of liquid.

Step 4 – Wait for about 30 seconds to allow the liquids to settle. You will notice three layers of liquid, first water at the bottom, then cooking oil in the middle and finally rubbing alcohol at the top.

Step 5 – As you observe the ruler in the glass, first notice that the ruler appears to get wider as soon as it enters the liquid within the glass cup. Immediately remove the ruler from the liquid to show that it really isn’t wider, it just appears to be when observed in the liquid.

Next, place the ruler back into the liquid and look very closely through the side of the cup. As you look closer, you will notice that the width of the ruler appears to change depending on the liquid it is in.

Do you know the why the ruler appears to change size? Find out the answer in the how does this experiment work section below.

Video Tutorial

Why does the Ruler Get Wider Experiment Video

How Does the Science Experiment Work

When the ruler is placed in the liquid it appears to get wider, but once we remove the ruler we see that it isn’t really wider at all. This is because of refraction. Refraction is the bending of light as it passes from one medium into another (ie. air to water, water to air).

As light travels through a given medium, it travels in a straight line. However, when light passes from one medium into a second medium, the light bends. Light refracts (bends) as it passes from one medium to the next because it travels at different speeds through those mediums.

When we look at the portion of the ruler that is outside the glass of water, light from the ruler travels in a straight path to the eye. However, when we look at the ruler within the water, the light is refracted as it passes from the liquid into the air. This causes the ruler to look wider even though it truly isn’t.

Furthermore, the type of liquid impacts refraction. This is because different materials have different indexes of refraction. A material’s index of refraction is a measure of how much light rays bends when they enter the material. Let’s look at the index of refraction in terms of water and air. Objects in water appear larger than objects in the air. The index of refraction for air is 1.00. The index of refraction for water is 1.33. This larger index of refraction for water means that the angle the rays of light reach your eyes is larger in water than the angle would be in the air. This increase in the angle size of light to your eyes makes the object look larger in water than how it looks in the air. The ruler looks larger still in rubbing alcohol and especially in cooking oil because the index of refraction for rubbing alcohol is 1.36 and the index of refraction for cooking oil is 1.47.

More Experiments that Show Refraction

Light Refraction Science Experiment – Watch in amazement as the arrow to changes direction.

Refraction of Light Science Experiment – Watch as the straw appears to bend in this experiment that shows refraction in action.

I hope you enjoyed the experiment. Here are some printable instructions:

Ruler Changes Size Science Experiment

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Materials

• Tall empty clear glass
• Water
• Cooking Oil
• Rubbing Alcohol
• Ruler

Instructions

1. Pour water into a tall clear glass. Fill it about 1/3 of the way full.
2. Next, carefully pour in cooking oil into the glass until the glass is a little less than 2/3 of the way full.
3. Slowly pour rubbing alcohol into the glass until the glass is almost full of liquid.
4. Wait for about 30 seconds to allow the liquids to settle. You will notice three layers of liquid, first water at the bottom, then cooking oil in the middle and finally rubbing alcohol at the top.
5. As you observe the ruler in the glass, first notice that the ruler appears to get wider as soon as it enters the liquid within the glass cup. Immediately remove the ruler from the liquid to show that it really isn’t wider, it just appears to be when observed in the liquid.
6. Next, place the ruler back into the liquid and look very closely through the side of the cup. As you look closer, you will notice that the width of the ruler appears to change depending on the liquid it is in.