Can you use a magnifying glass and moonlight to light a fire?
Ilana Taieb Hagerty / 8th grade
An answer to a crazy question, every month.
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At first, this sounds like a pretty easy question.
A magnifying glass concentrates light on a small spot. As many mischevious kids can tell you, a magnifying glass as small as 6.4 square centimeters in size can collect enough light to start a fire. A little Googling will tell you that the Sun is 400,000 times brighter than the Moon, so all we need is a 2,560,000-square-centimeter magnifying glass. Right?
Wrong!
Wrong. Here's the real answer: You can't start a fire with moonlight no matter how big your magnifying glass is.
First, here's a general rule of thumb: You can't use lenses and mirrors to make something hotter than the surface of the light source itself. In other words, you can't use sunlight to make something hotter than the surface of the Sun. We can prove this with thermodynamics:
Lenses and mirrors work for free; they don't take any energy to operate. If you could use lenses and mirrors to make heat flow from the Sun to a spot on the ground that's hotter than the Sun, you'd be making heat flow from a colder place to a hotter place without expending energy. The second law of thermodynamics says you can't do that. If you could, you could make a perpetual motion machine.
The Sun is about 5,000°C, so our rule says you can't focus sunlight with lenses and mirrors to get something any hotter than 5,000°C. The Moon's sunlit surface is a little over 100°C, so you can't focus moonlight to make something hotter than about 100°C. That's too cold to set most things on fire.
"But wait," you might say. "The Moon's light isn't like the Sun's! The Sun is a blackbody—its light output is related to its high temperature.