Light For Heat
What you need
- Two identical wide-mouthed containers — two tin cans, two small steel mugs or two glass tumblers. Same size, same material, same colour.
- Water, enough to fill both containers to the same level, poured from the same jug so both start at the same temperature.
- A thermometer reading from 0°C to 100°C; hands will do if there is no thermometer. For the second half, black paint that will not dissolve in water, or tar, and a brush.
The manual states these. What it does not state is not here.
How it runs
The teaching sequence as the manual gives it.
-
01
Take two identical wide-mouthed containers — two tin cans, two small steel mugs, two glass tumblers. They must match in size, in material and in colour. If they differ in any of those, the result proves nothing, because the class can always blame the container. -
02
Fill both with the same amount of water, poured from the same jug so that both start at the same temperature. Leave the mouths open. -
03
Measure the temperature of the water in both containers with a 0°C to 100°C thermometer and write both readings on the board before you start. Put one container in full, direct sunlight. Put the other in shade close by — same place, same air, just out of the sun. Not indoors: the only difference between the two must be the sunlight falling on one of them. -
04
Wait one hour. Measure both again and write the new readings beside the first pair. The one in the sun is warmer. Ask the class what warmed it. Nothing touched it and nothing burned. Sunlight crossed empty space, arrived as energy, and the water turned that energy into heat. -
05
If there is no thermometer the activity still works. Have students dip a finger into each container and say which is warmer. Send several of them up rather than one, and write down what each says — a single opinion about warmth is easy to argue with, and six agreeing is not. -
06
Now the second half. Paint the inside of one container black, using a paint that will not dissolve in water, or tar. Leave the other one exactly as it is. Let the paint dry completely before going on, or it will come off in the water and ruin the test. -
07
Fill both containers with the same amount of water again, from the same jug. This time put both of them out in the sun, side by side, for the same length of time. The only difference now is the colour inside. -
08
Measure both after one hour. The black one is warmer. Ask why. A black surface soaks up nearly all the light that lands on it and turns it into heat, while a shiny or pale surface throws much of it back. That single fact is the solar cooker: a black vessel, a lid that lets the light in, and patience. It is also why a black water tank on a roof gives hot water by afternoon in May, and why white clothes are cooler than dark ones in summer.
Where this comes from
Bridge the Gap is a compilation. This is what the manual records for this session.
The manual prints no source for this session. It is published here as Swechha has used it, adapted over years of classroom work; if you recognise its origin, tell us and we will credit it.
Before you start sets out where the compendium as a whole comes from.
Next
Where to go from here.