How to make a disappearing glass?

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Disappearing glass and refractive index

Introduction

Your children will be surprised and amazed with the result. In this post we compare refractive index of water and refractive index of glycerin. As a matter of fact glycerin has a similar refractive index as glass. Therefore, we used this physical property to perform experiment which your children will love. Firstly, the main advantage of this experiment is that you can explain to the child the properties of light and matter. Secondly, it could be a fantastic introduction to optics.

In our experiment we use:

  1. Water
  2. Glycerin
  3. 2 glasses
  4. Test-tube 

Experiment execution

This experiment should be executed using one glass filled with water and a second glass filled with glycerin.

Attempt 1 – Glass filled with water

First we show to the children empty test-tube and we put it into the glass filled with water. We can ask them: Can you see the test tube? Answers can differ but in general it should be “yes, we can see it”.

Later on, you can pour the water inside the test-tube (the one in the glass of water) and repeat the question. With high probability, the result will be a visible test-tube.

Attempt 2 – Glass filled with glycerin

During the second attempt we show to the children empty test-tube and we put it into the glass filled with glycerin. We can ask them: Can you see the test tube? Answers can differ – Atena says she sees everything all the time 😉 – but in general it should be “yes, we can see it”. Probably a little bit harder than during the disappearing glass Attempt 1, but still possible to see.

Later on, you pour the glycerin inside the test-tube and repeat the question. With high probability, the result will be that the test-tube is not visible at all.

Explanation for pros

Disappearing glass happens, because glycerin has the similar refractive index as glass. During our first attempt we could still see the test-tube (made of glass) as refractive index of glass differs significantly from refractive index of water. For instance, we can take a look at the below refractive index table.

MaterialGlycerinGlassAirWater
RI1.47311.51.00031.33

When we put empty test-tube to glycerin, we see it, however when we put glycerin inside, the test-tube disappears. It happens due to the fact that less reflected light is coming back to our eyes – majority of it goes straight through the filled tube. Do you need more data? You can check it here.

What is refractive index?

Refractive index of material describes how fast light propagates in the material. For example if refractive index of glycerin is 1.4731, it means that light travels 1.4731 faster in vacuum than in glycerin.

Interesting facts

Optics is a very interesting study. Below you can find one interesting fact.

Do you know why cats have vertical pupils? 

Due to the vertical pupil, cats can regulate the amount of light entering the eye through the movement of the eyelids, which are perpendicular to the pupil, without the need for accommodation of the eye. It is very important for them as they typically hunt at night.

Summary

If you liked this experiment, please do not hesitate to try perform this with different materials. Some suggestions below:

Glass fillingTest-tube filling
WaterAlcohol
MilkAlcohol

Can you please find refractive index of alcohol and milk and compare it with refractive index of glass? What will happen based on your knowledge gathered during this lesson?

Please leave the comment below. We are curious what are your suggestions and results.

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