# Producing heat(fire) using magnifying Glass

Hello! :D I wish to know how is Magnification related to Focal Length or Focal point in terms of magnifying lenses? We are researching how magnifications affect the heat produced by magnifying lenses, unfortunately, I found out that it's mostly the size. does this mean that the magnification really has no effect on the heat produced? or What other factors affect it?

When you hold a magnifying glass in the sun and place something at its focal point, it creates a high temperature because it takes all of the radiation that hits the large area of the magnifying glass, and focuses it into a tiny point, making a high concentration of heat. You can focus light at the focal point of any magnifying glass, but the larger the area of radiation you can focus onto the point (the larger the magnifying glass), the higher the concentration of heat at the focal point.

The maximum angular magnification (MA) of a magnifying glass is given (approximately) by:
$MA=\frac{25 cm}{f}$
f is the focal length, meaning that higher magnification is achieved by a shorter focal length.
A magnifying glass with a high magnification will mean that the focal point is closer to the glass, but this does not effect the concentration of heat at the focal point.

HallsofIvy
Homework Helper
The point is that there is NO "magnification" involved here at all. It is simply a matter of focusing all the light that passes through the glass on a single point. And how much that is depends on the area (and so radius) of the glass.

sophiecentaur