# Burning wood with a magnifying glass

## Main Question or Discussion Point

Hi,

In our class, we were asked whether it's practical to use a magnifying glass to focus sunlight to burn wood. One question really bothers me.

Could somebody please explain to me how the focal ratio (focal length of converging lens divided by its diameter) affects the temperature of the wood? It seems to me that only the collecting area (cross-sectional area) of the lens matters. I mean, no matter how long the focal length is, the light rays are still focused to the same point, right?

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The light is not actually focussed to a point. What you get is an image of the sun. The size of the image will be (image distance/objectdistance) sun dia. Since the image distance is very close to the focal length of the lens, a shorter focal length will produce a smaller image and a hotter spot on the wood.

Measuring magnification

How to Measure magnification
How can I see how strong a magnifying glass is?

I bought this magnifying glass http://www.liangdianup.com/inventory/189901.htm [Broken] and I want

to know how to test it to see how strong it is. I hear a lot of people talk about

magnifying and how strong the magnification is, but I would like to know the true

magnification of my magnifying glass. I have a few of them and some seem stronger then

others. How can I rate these? How can I pin a correct number on mine? How do the companies

that make these come up with these numbers.

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