Challenge: Make an Underwater Torch

With access only to a basic tool kit, some reels of wire, pots and pans, rusting kitchen equipment and a car battery would we be able to make a light at all let alone a torch that worked underwater?

Firstly, how is light created?

Whether using fire, electricity or even a chemical reaction, you are essentially doing the same thing: giving atoms energy and getting them 'excited'. The energy absorbed by the atoms is taken up by the electrons that surround an atom's nucleus.

Used with permission The energy carries the electrons further away from the nucleus than normal and the atom is in an 'excited' state. When the electrons fall back to their ground state, closer to the nucleus, energy is radiated away again.

Electrons can give off different kinds of electromagnetic radiation when they fall back to their ground state but we're particularly interested in getting them to emit radiation within the visible spectrum.

How can we get the atoms excited?

One way to get atoms in a metal excited is by putting an electric current through them. If you put an increasing current through a piece of metal it will eventually get red-hot. If you up the current a bit more, it will eventually get white hot. When a metal is glowing with heat, it is described as being incandescent.

Why is light given off?

In a red-hot object, the atoms are just getting enough energy to begin emitting the light that we can see. Red is the first visible light to be seen because it is the colour which requires the least energy for emission.

Once you apply a big enough current, you will energize all the electrons to such an extent that light of all colours is being generated. These colours mix together to appear white.

Used with permission We decided to use a light bulb as the basis for our underwater torch.

The concept of the light bulb is quite simple. An electric current flows through the filament from the source. The filament emits energy in the form of visible light and heat. The glass bulb helps to protect the wire and enables it to be surrounded with a gas other than air and/or a partial vacuum. Changing the gas around the filament can serve to make it last longer. So, in essence, a light bulb is simply a small metal wire that is glowing with heat.

Why does the filament glow?

Electricity flowing through the filament is essentially just electrons moving along the wire. The movement of these electrons isn't smooth: as well as moving in one direction, they are also bouncing around, hitting each other.

The nuclei of the atoms of the wire form a lattice that the electrons have to move across. The material is described as having a 'resistance' to the electron flow. The more resistance the metal offers, the more electrons bash each other and the nuclei and the hotter the material gets.

The heat energy gets other electrons, still trapped around the metal atoms' nuclei, excited. The energy pushes the electrons further away from the nuclei of the atoms. As the electrons fall back, they emit photons of light.

Used with permission Used with permission

How can we make our own light bulb?

Firstly, a resistor was put in the circuit in series. The resistor was a very long bit of wire that we knew wasn't great at conducting a current. We gradually reduced the length used, then took it out of the circuit when we were sure it was safe.

We tested out which wires glowed the brightest and lasted the longest. We began by using a 50cm piece of wire coiled up and gradually reduced the length until it glowed.

The shorter the length, the more brightly it glowed but the more quickly it burnt out. To increase the brightness we used two filaments in the same glass jar.

Used with permission Used with permission

Now that we have a filament, how do we remove the oxygen from the light bulb?

Oxygen can be removed by burning a candle inside the sealed bulb. Having ensured that the candle wasn't touching the filament, a current was passed through using a car battery or several batteries in parallel. The candle going out indicates that the oxygen has been used up.

How do we turn our light bulb into an underwater torch?

Once the bulb was working nicely above water it was sealed very carefully and tested for air tightness underwater. It was very important to ensure that the sealant used was effective - we used sticky cherry and other plant resins to seal the bulb. Once we were sure that there were no leaks, the leads were connected to the terminals of the battery.

Used with permission Used with permission

Did the torch work?

The torch was lowered into the water off Carriacou and proved to be a glowing success - the fish weren't quite so impressed though!

Web Links

The BBC and the Open University are not responsible for the content of external websites.

How Light Works - from the How Stuff Works site

Make your own Salt Volcano - from the Exploratorium website

Incandescent Light Bulbs - from the How Things Work, University of Virginia site

Books

Advanced Physics by Tom Duncan, John Murray