Sunlight on one small spot

The idea

A concave mirror facing the Sun concentrates reflected sunlight onto a small bright spot that can become hot enough to ignite paper; devices that concentrate sunlight with mirrors and lenses are called solar concentrators.

Light in a small area

"A concave mirror converges a light beam. So wouldn't light get concentrated in a small area?" A concave mirror faces the Sun, and a piece of paper is held in front of it.

What do you think appears on the paper?

• A small bright spot that gets hot
• A clear picture of the mirror
• Nothing at all

Find the hot spot

Move the paper nearer to the mirror or farther from it. Find where the spot of light is smallest and brightest.

Solar concentrators

A concave mirror facing the Sun concentrates reflected sunlight onto a small bright spot. The spot can become hot enough to ignite paper. Devices that concentrate sunlight using mirrors and lenses are called solar concentrators.

Safety: never look at the Sun or at the bright spot. Try this only with your teacher.

• Concentrated sunlight is used for | How
• Making electricity | It heats a liquid to make steam
• Large-scale cooking | It gives a lot of heat
• Solar furnaces | They get so hot that they can even melt steel

Notes

A concave mirror facing the Sun concentrates sunlight onto a small bright spot that can get hot enough to ignite paper. Devices that concentrate sunlight with mirrors and lenses are solar concentrators.

Check yourself

Why does the paper get hot at the bright spot?

Where is the paper hottest?

What are devices that concentrate sunlight using mirrors and lenses called?

What can the heat from a solar concentrator be used for?

• The mirror concentrates sunlight from its whole surface onto that small spot — correct. Yes. The same sunlight falls on a much smaller area.
• The mirror makes its own heat. A mirror makes no heat. It reflects sunlight onto a small area.
• The paper pulls in the Sun's light. Paper does not pull light. The mirror converges the light onto it.
• Right next to the mirror, where the spot is biggest. There the light is spread over a big area, so it is less hot.
• Where the reflected rays meet and the spot is smallest — correct. Yes. All the light falls on the smallest area there.
• Anywhere: the heat is the same everywhere. The same sunlight on a smaller spot makes it much hotter.
• Plane mirrors. A plane mirror keeps parallel rays parallel. It does not concentrate them.
• Periscopes. A periscope uses mirrors to see over things. It does not concentrate sunlight.
• Solar concentrators — correct. Yes. They concentrate sunlight into a small area.
• Heating a liquid to make steam for making electricity — correct. Yes. Large-scale cooking and solar furnaces use it too.
• Making ice. Concentrated sunlight gives heat. It would melt ice, not make it.
• Making a room darker. A solar concentrator gathers light and heat. It does not make things darker.