Why can’t you make a fire using a magnifying glass and some “special” flashlight or moonlight?

Jaroslav Kores, Ph.D.

It would certainly be possible but we would need either a better flashlight or a bigger magnifying glass. About 1,000 W/m2 of energy reaches the Earth from the Sun. If the Moon reflects roughly 20% of the falling sunlight, we would need a 5x better magnifier (i.e. a magnifier with a 5x larger surface, i.e. a 2.3x larger radius). We should place the magnifying glass perpendicularly to the rays that go from the Moon. In the case of a flashlight, we would need to put (ideally halogen or classic) bulbs with a total power of 1,000 W so that all the light from them goes in one direction and illuminates an area of 1 m2. Or use half the bulbs and the area they would illuminate would also be half or a quarter… and so on up to the size of the magnifying glass.

If we wanted to ensure success even more, it would be good to use a hollow mirror — because the light is reflected from it, there will be no energy losses caused by the passage of light through the glass.

Or we can use a slightly more powerful laser — with it, the energy is already focused into a narrow beam.

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