how big of a magnifying glass would it take to start a fire with moonlight?

Answer 1

I think it would be impossible. If you fill the lense of a telescope with a bright moon, that is the same effect of using a magnifying glass and you can't even damage your eye.

Answer 2

The key to this is how hot the Moon is versus how hot the Sun is. The Sun's surface temperature is about 5600K, while the Moon's surface temperature is about 360K. This means that the Sun is over 15 times as hot as the Moon, and so the light leaving its surface would carry that much more energy, all things being equal. This means that you would need 15 times the area of a lens used to start a fire with sunlight.

However, you would not use a regular lens because they are inefficient and you might not be able to focus the light to a small enough spot. You would use a special type of "non-imaging" optical lens that simple concentrated the light without creating an actual image of the Moon. Then you would be able to do it.

How can we prove this? Telescopes looking at other planets measured their temperatures using thermocouples (to measure heat) by focusing the light from those planets onto the thermocouples. This would raise the temperature of the thermocouple slightly, so it is possible to get heat concentrated this way, just not much of it usually.

Answer 3

What actually ignites the fire? The focused light, or the focused heat? I would say the focused thermal energy form the sun actually starts the fire. So, the moon, absorbing most of the heat (since the temp in the moon is very hot) is reflecting on a very small amount, and you wouldn't be able to start a fire with a magnifying class the size of Texas.
Could anyone else chime in about how light and heat from the sun are carried to earth? that would help to answer the question about focusing it.

Answer 4

Moonlight has not nearly enough magnitude, and a large magnifying glass wouldn't help. You also have to keep in mind that a large magnifying glass would be very heavy and prone to losing the correct point of focus.

Answer 5

The moon has no energy of its own, in fact its very cold ! almost has cold has space its self, approaching absolute zero..
what you see when you look at the moon is (visible light rays from the sun) , bouncing off the surface of the moon while or on special times of the year reflected light from the earth.. (earth glow, when the moon appears to gain a red tint) there is no way you could start a fire while using any sized magnifying glass just by using moon light!.

Answer 6

Light does not start fire, with any size magnifying glass. If you focus infrared rays, you may be able to start a fire, but the moonlight is reflected sunlight, and the moon absorbs the infrared.

Answer 7

I am not sure people are too aware of the difference in the size and power : Sun v Moon

In pure volumetric terms, the sun is nearly 70 million times the size of the moon.

The sun outputs energy due to it being a continuous nulcear fusion process, much like a continuous hydrogen bomb, the bomb itself being 860,000 mile across.

The moon is totally dead. It has no energy output at all. It is simply a nice mirror for reflecting a tiny, tiny portion of the massive amount of light that comes from the sun all the time.

Finally, even though the sun it 400 times as far away as the moon, it sheds several hundred thousand times as much light on us as does the moon.

The reason you do not notice this awesome difference in light, is that your pupils close up in the day, and dim a lot of the light. At night they open right up and make the moon look bright and silvery.

Hope that answers your question.

Answer 8

Moon "light" is not truly light, but rather a reflection of the light produced by the sun. It would not be the size of the magnifying glass that would start the fire, but rather an intensification of that reflection that could start a fire. *smile*

Answer 9

Frankly I think that would be impossible. No matter how big it was, there still wouldn't seem to be enough heat to actually accomplish that.