Disappearing Beaker (Refractive Index Matching)

Making a glass object invisible by immersing it in a liquid of matching refractive index

Classroom Demonstrations Beginner (Middle School) 15 min ~$8.00

Objective

Demonstrate the concept of refractive index by making a glass object disappear in a liquid that matches its refractive index.

Background

We see transparent objects like glass because light bends (refracts) at the surface where the glass meets air. The degree of bending depends on the difference in refractive index between the two media. If a glass object is placed in a liquid with the same refractive index, light passes through without bending, and the object becomes effectively invisible. Pyrex glass (n ≈ 1.474) nearly matches vegetable oil (n ≈ 1.47), creating a stunning visual effect.

Safety Warnings

  • Vegetable oil is slippery — clean up spills immediately to prevent falls
  • Glass objects can break — handle with care

Required PPE

goggles

Materials

  • Vegetable oil (or glycerol) (500 mL)
    Refractive index ≈ 1.47
  • Pyrex glass beaker or rod (1)
    To disappear
  • Regular glass rod (1)
    For comparison (different n)

Equipment

Large glass beaker (1 L) or clear container

Procedure

1

Fill the large beaker about 2/3 full with vegetable oil.

2 min
2

Show the Pyrex glass object to the audience. Note that it is clearly visible in air.

1 min
3

Slowly lower the Pyrex glass into the vegetable oil. As it submerges, it gradually becomes invisible.

3 min Lower gently to avoid splashing
4

For comparison, lower the regular glass rod into the same oil. It remains visible because its refractive index does not match.

3 min
5

Discuss: light refracts at boundaries between materials with different refractive indices. When n values match, no refraction occurs and the object seems to disappear.

6 min

Expected Results

The Pyrex glass becomes nearly invisible when submerged in vegetable oil. Only slight distortions at the edges may be visible. The regular glass rod remains clearly visible in the same oil.

Cleanup

Remove glass objects and wash with dish soap. Oil can be saved for reuse or disposed of in general waste. Wipe up any oil spills.

Details

Category
Classroom Demonstrations
Difficulty
Beginner (Middle School)
Duration
15 min
Est. Cost
$8.00
Steps
5
Materials
3

More Experiments

Burning Magnesium in Dry Ice 25 min Chemiluminescence with Luminol 30 min Density Column Rainbow 30 min Elephant Toothpaste 20 min Instant Freeze: Supercooled Water 120 min Iodine Clock Reaction 45 min Non-Newtonian Fluid: Oobleck 20 min Oscillating Briggs-Rauscher Reaction 40 min Sodium and Water Reaction 25 min Thermite Reaction 30 min

Frequently Asked Questions

What is the objective of Disappearing Beaker (Refractive Index Matching)?
Demonstrate the concept of refractive index by making a glass object disappear in a liquid that matches its refractive index.
How difficult is Disappearing Beaker (Refractive Index Matching)?
This experiment is rated as Beginner (Middle School). It takes approximately 15 minutes to complete.
What safety precautions are needed for Disappearing Beaker (Refractive Index Matching)?
Key safety precautions include: Vegetable oil is slippery — clean up spills immediately to prevent falls; Glass objects can break — handle with care.
What materials are needed for Disappearing Beaker (Refractive Index Matching)?
The main materials required are: Vegetable oil (or glycerol), Pyrex glass beaker or rod, Regular glass rod.
What results should I expect from Disappearing Beaker (Refractive Index Matching)?
The Pyrex glass becomes nearly invisible when submerged in vegetable oil. Only slight distortions at the edges may be visible. The regular glass rod remains clearly visible in the same oil.