Totally Reflected

sinc-e 2013
Hiša eksperimentov
Ljubljana, Slovenia

Do and observe!

Pick a light bulb that isn't submerged in water and observe it closely. Then rotate the wheel a little with the buttons below the video, so that the bulb is completely under water. Does the bulb seem any different than before being submerged?

Repeat the procedure with the other bulbs. Do you notice the same change in all of them?

loading video feed ...

What's going on?

The bulbs numbered 1, 3 and 5 appear different when they are under water: it seems as if there was an additional smaller bulb inside the larger one, and a reflection of the surroundings can be seen on the outer surface of the bulb as if in a mirror. This phenomenon however cannot be seen with bulbs numbered 2, 4 and 6.

The bulbs in this experiment are not connected to electric power. We can see them because of the ambient light reflected into our eyes. If the light rays enter our eyes from a different direction, the image of the light bulb is different, even though the light bulb itself hasn't changed at all.

Light can change direction while travelling through different transparent media. Light bends differently when travelling from glass into water than when the bulb is surrounded only by air. This is the reason the submerged light bulbs look different than the ones not submerged.


Light rays can bend on their way through different transparent media. They can even be reflected on the boundary between two transparent substances, although there are no mirrors there. This phenomenon is known as total internal reflection. The phenomenon can be observed in light bulbs numbered 1, 3 and 5 with smooth glass.

The light bulbs numbered 2 and 4 have a coarse surface, so light gets scattered on the surface and doesn't enter the glass like it does with a transparent surface. This is the reason we don't observe the phenomena we noticed with the other light bulbs.

The compact fluorescent lamp (number 6) has a transparent glass surface but the glass is thinner and the surface more curved, so the effect is not as noticeable as with light bulbs 1, 3 and 5.

Did you know?

Television, telephone and internet information is mostly transmitted by light impulses travelling along optical fibres. Total internal reflection plays an important part in this technology by ensuring that light remains locked inside the optical fibre all the times.

Icons made by Freepik from