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This problem occurred to me the other day, so I decided to see if I could work it out, but am stuck.

How much heat can be generated (in therory) using an ordinary magnifying glass to focus the sun's rays on a sunny day?

Here's what I'm working with:

A decent approximation for the sun's power on the surface of the earth is about 1200 W/m^2.

I'm assuming a glass lens about 3" in diameter. Changing units and figuring the area gives 0.00456 m^ for the area of the magnifying glass.

The magnifying glass will intercept roughly 5.5 W/m^2 available from that 100 W/m^2 the sun is providing. I think I'm satisfied with this so far, but what I'm having trouble reckoning is what to do next.

I believe the kindling temp of paper is about 450 F, and I know paper easily burns by putting a mag. glass to it, and I still think it's cool!

Anyway, seems to me that the smaller the spot the mag glasss gives the greater the heat (within reason) that can be generated.

I'd like to get a little guidance please. This is getting under my skin and it is frustrating to have my mind bouncing off this without being able to proceed.

Anyone have some input or ideas?

Thanks!

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# How much heat from a magnifying glass?

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