Demo Video: The Vanishing Eyedropper

Video Transcript

In this video, we will be observing how materials with similar refractive indices can be used to make one of the objects seem invisible!

This eyedropper is made of glass. The liquid in the bottle has a very similar refractive index to that of the glass used in making the eyedropper. This means that both the glass and the liquid distort light by bending light rays in a very similar way. This fact can result in a rather surprising phenomenon.

When we lower the eyedropper into the bottle full of liquid, we can still see it. This is because the eyedropper is currently full of air. Air has a different refractive index to that of the glass that forms the eyedropper, so it bends light slightly differently. But let’s see what happens when we fill the eyedropper with the liquid in the bottle.

Now, the bottom of the eyedropper is filled with the liquid. Remember, this liquid has the same refractive index as the glass that is used in making the eyedropper. Because of this, the light entering the bottle from behind is bent in exactly the same way by the liquid in the bottle, the glass making up the eyedropper, and the liquid in the eyedropper before it leaves the bottle again and enters our eyes. Therefore, we cannot see a difference between the liquid and the glass. It looks to us like the eyedropper is made of the same material as the liquid, and it cannot be told apart from the liquid.