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I would like to know how long would it take for a small or medium sized mag. glass to heat a black painted metal particularly aluminum since I can't do the maths.

Andrew Steane
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NOIZ101
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    This will depend on a lot of factors - precisely how big a "small" or "medium" sized glass is, what the local weather conditions (I imagine you're using the Sun as the light source) are, the time of day, the mass of the metal piece, how "black" the black coat is, and probably many more. So there is not a unique answer here. – The_Sympathizer Dec 03 '18 at 09:02
  • The case is this;a soda can tightly selaed and painted black at the bottom and the magnifying glass is 70mm.Temperature is 30 degrees Fahrenheit and the liquid inside the can is water. – NOIZ101 Dec 03 '18 at 13:24

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In your comment, you say you are trying to heat the following object...

a soda can tightly selaed and painted black at the bottom ... inside the can is water.

using a 70mm magnifying glass.

In this case, the magnifying glass will probably not raise the average temperature of the can at all! The magnifying glass focusses a 70mm circle of sunlight onto a smaller circle. This means that most of your can will be in shadow. In other words, the glass will make your can hotter at the focal point but cooler far from the focal point.

Most of the light focused by the glass would have hit the can anyway. On the other hand, the glass would be very useful in heating a very small object.

James
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