Focal point of a magnifying glass?

Question

given a 3 inch diameter magnifying glass, how hot would the focal point be in average sunlight? What would be the diameter of the focal point?

Answer 1

A lot depends on how precisely the glass is made, and local conditions.

I don't have an analytical answer, but my experience from playing with these things as a boy says that the focal length of such a glass is usually about twice its diameter, and the spot of the sun's image will be 1 to 2 mm in diameter.  A glass this size is hot enough to melt 60/40 solder, so I'd wager it gets to 700 F or so.  Whether you get that depends on how much wind there is, how hot the air is, and whether the sun is directly overhead or not.  In a secluded, windless valley in Hawaii, a 3" glass was generally not enough to easily start a piece of newspaper burning.  A 4" glass did it easily.