Invisible Glass

Age

Middle School, High School

Format

Stage Show, Hands-on

Materials

   Two glass containers (for a hands-on show, you can use a small beaker).
   Two glass rods (e.g. stirring rods or test tubes) taller than the containers
   Enough water to fill the container
   Enough oil to fill the container (baby oil works well, vegetable oil is also ok) 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Although it is harmless, to avoid a mess you should not let the audience touch the oil or water. If you let the audience handle the glassware, supervise them carefully.

Preparation

Fill one container about 2/3 full with water and stick a glass rod in it. Stick the second glass rod in the other container and be ready to fill it with oil.

Demonstration

1. Show the audience the empty glass container.

2. Show the audience the container with water. Point out that the glass rod looks bent due to refraction.

3. Pour the second container about 2/3 full with oil. The glass rod should appear invisible through the oil. Note: if you use a hollow glass rod (i.e. test tube), it is important to fill the test tube with the oil so that there is no air for the light to pass through.

What to Say

Have you ever wondered why you can see things that are transparent? Shouldn't they be invisible?

Perform step 1 of the demo. Why can you see this glass? It's because the light from behind the glass gets bent when it travels through the glass! We call this refraction - this bending of the light causes a distorted picture behind the glass and helps you figure out that the glass is there.

Perform step 2 of the demo. Water and glass bend light by different amounts - they have different indexes of refraction. The light coming from just the water is bent a little bit, but the light coming from behind the glass is bent even more - it's even more distorted. That's why you can see the glass in the water!

Perform step 3 of the demo. Why does the glass look invisible? This oil actually has about the same index of refraction as the glass. Now everything in the entire container bends light the same way. As far as the light is concerned, the oil is no different from the glass, so you can't tell where the oil stops and where the glass begins!

A longer demonstration format to introduce the subject of wave optics:

Light is a very complicated subject in science. Light acts like both a wave and a particle - that means it can move in a straight line, like a ball that has been thrown, or it can move through space like a wave in the ocean. It's easy to picture light as a particle, a ball, moving through space, but does it seem strange to you that light would act like a wave? In fact, we see light act like this every day!

Perform step 1 of the demo. Have you ever wondered why you can see things that are made from glass? After all, glass is transparent - shouldn't it be totally invisible?

The reason you can see glass is because it has a different index of refraction than air. The index of refraction (n) measures how much a material can make light waves slow down. The speed of light in a material equals the speed of light in vacuum, 3x10^8 m/s, divided by n. Air has an index of refraction of about 1, meaning it hardly slows down light at all. Glass has an index of refraction of about 1.5, meaning it slows down light by about 33% (0.33c=c-c/1.5)!

When light slows down, it bends! This is called refraction. The glass has a large index of refraction, so it bends light a lot. That's why objects behind the glass look distorted - the glass is bending the light from those objects before they read your eye. We use this distortion to figure out when there's a piece of glass in front of us.

We see refraction in ocean waves, too! Ocean waves bend when they approach the shore: "The part of the wave crest closer to shore is in shallower water and moving slower than the part away from the shore in deeper water. The wave crest in deeper water catches up so that the wave crest tends to become parallel to the shore." Have you ever seen a wave bend as it approaches the beach? No? Exactly! Refraction guarantees that, if there is a steady slope in the sand as it goes closer to shore, the wave will always arrive parallel to the beach!

Perform step 2 of the demo. Of course, water bends light in a way similar to the glass. It has an index of refraction of about 1.33, so it slows light down by about 25%. Since this isn't quite as much as the glass slows the light down, light isn't bent as much by water as it is by glass. That's why we can see a glass straw in this water. Light from behind the water looks distorted - light from behind the glass straw looks even more distorted!

But what if we didn't use water, but instead used some liquid that has the same index of refraction as the glass? What do you think would happen? What would it look like? Let's find out! We can use oil, which has an index of refraction about the same as glass, n~1.5

Perform step 3 of the demo. Look, the glass rod disappears as I pour in the oil! Now everything in the beaker has the same index of refraction - both the oil and the glass rod. Therefore light is bent exactly the same way throughout the entire beaker. That means the background image is distorted the same way everywhere. Since there's no difference between the background image in the part with just the oil and the part with the glass, we can't tell where the glass rod is!

Why It Is

Why does light slow down in materials? The configuration of atoms and their associated charged particles, protons and electrons, in a material determines how it interacts with electromagnetic radiation (light). The electromagnetic wave is composed of oscillating electromagnetic fields, which cause the charges to oscillate. The oscillating charges then emit more electromagnetic waves. This is essentially the anbsorption and reemission of photons. Depending on the material, the charges may not be able to oscillate in sync with the incoming electromagnetic wave, so the outgoing electromagnetic wave may be slowed down.

Why does the change in speed cause light to bend? If you're thinking about light as a wave, consider that the part of the wave reaching the new medium at a smaller angle will reach it sooner than the part at a larger angle. See the diagram. The nearer part will then slow down before the other part of the light, so it will drag the light wave towards the light entering at the smaller angle. This is like a toy wagon being dragged from pavement to grass at an angle. One wheel will reach the grass first and will slow down earlier. If the other wheel keeps moving at the same speed on the pavement, the wagon will turn. The situation for ocean waves is similar.

How exactly does the light bend? If light enters a material with a high index of refraction at an angle, it will bend so that it moves through the material at a smaller angle (i.e. it bends towards the normal of the surface. the normal is a line perpendicular to the surface). This effect is described by Snell's law.

The audience will probably be familiar with the effect of a straw looking bent or disconnected when placed in water. This happens for a very similar reason - water has a different index of refraction than air. The light from the part of the straw in air heads straight for you eye, while the light from the part of the straw in water is bent before it gets there. That's why the two parts of the straw appear to come from different places.

You might be asked why.

Real Life Examples

Straws in a drinking glass.

Lenses, like the ones in your glasses, camera, or microscope work by refraction.

Ocean waves refract, too!