Why Can’t We Magnify Light From The Moon To Make Fire?

But is it possible to harness light from the Moon in the same way? When the Moon is full, could you focus this light and use it to produce heat here on Earth?

First up, you can focus the Moon’s light, just as you can the Sun’s. Nothing is stopping you from taking a magnifying glass and focusing it down to a small area, just as you can with light from the Sun that has not yet reflected off of the Moon. But if you try it with a standard Sherlock Holmes magnifying glass, you will find that the focused moonlight is still pretty cool, temperature wise.

Thanks to the good old second law of thermodynamics, which states that heat always flows from hotter to colder areas (and a whole lot more than that), we know that it is not possible to focus light to produce hotter temperatures than the light’s source. Since a lens is merely focusing light, and no energy is being added by it, if it were to make the Earth/paper/small plastic army men hotter than the Sun, that would mean that you have made energy flow from a colder point to a hotter one, breaking the laws of thermodynamics and getting some free energy in the process. 

That’s simply not possible according to all known laws of physics, and is often expressed as “there’s no such thing as a free lunch”.

The Sun, as you may have gathered, is pretty hot. Its surface clocks in at around 5,500 degrees Celsius (10,000 degrees Fahrenheit). While it would take a big lens to focus the light here on Earth to anything approaching those temperatures, you can get plenty of heat out of it to make a fire. The Moon is a lot cooler, with surface temperatures at the equator reaching a maximum of around 121 degrees Celsius (250 degrees Fahrenheit) in daylight. Paper burns at around 480 degrees Fahrenheit (250 degrees Celsius), so good luck starting your Moon fire.

While you may think that a larger lens would be able to produce fire with moonlight as it can collect more light, or focus it down to a really tiny point, this wouldn’t work either, even if your lens were truly gigantic. 

“It turns out that any optical system follows a law called conservation of étendue. This law says that if you have light coming into a system from a bunch of different angles and over a large ‘input’ area, then the input area times the input angle equals the output area times the output angle. If your light is concentrated to a smaller output area, then it must be ‘spread out’ over a larger output angle,” xkcd cartoonist, science communicator, and author Randall Munroe explains.

“In other words, you can’t smoosh light beams together without also making them less parallel, which means you can’t aim them at a faraway spot.”

While a fun thought, it will never be possible to use a magnifying glass and the Moon to burn your big pile of suspicious documents. Alas, you must wait for the day.