Using magnifying lens to heat up and object

[m25]

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When using a magnifying lens to burn a paper below the lens using energy from sun, how far should the paper be from the magnifying lens for the heating to occur most quickly?

 

[Cawolf]

The focal point.

The question asks about a single convex (converging) lens.

1/f = 1/o + 1/i

We want to focus the sun's rays, this will occur at the image distance.

The object distance is infinity and 1/infinity is zero (well the limit is....).

So 1/f = 1/i and f =i

The image will form at the focal point as a real image.

 

[mauroz]

It depends on the magnifying glass, but you would simply have to calculate the focal length which is where the energy from the light hitting the glass will come to one point.