Can You Start a Fire with Moonlight and a Magnifying Glass?

Equipped with only a magnifying glass and the light of the Sun, it’s pretty easy to start a fire.¹ So, with a much bigger glass, could you start a fire with moonlight?

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.

There are lots of ways to show why this is true using optics, but a simpler — if perhaps less satisfying — argument comes from thermodynamics:

Lenses and mirrors work for free; they don’t take any energy to operate.[2] 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.

In a better world, Randall Munroe would be writing middle school science textbooks.

1. A few summers ago when I was showing my kids how to do this, I started futzing with the small pile of tissue paper and pine needles we’d assembled and forgot about the magnifying glass I was holding…until I felt a searing pain in my leg and looked down to see a bit of smoke curling up from my slightly toasted thigh. Kids, do as dad says, not as he does.↩

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