Burning wood with a magnifying glass

[einai]

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?

Thanks in advance!

[Bonfield]

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.

[hellogreen]

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