The Telegraph
Since 1st March, 1999
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Why does paper burn under a magnifying lens'

KnowHOW team explains: A magnifying glass is a convex lens ' both its surfaces are curved outward. When light ray passes through such a lens, having arrived along a direction parallel to its axis, they converge at a point called the focus. The distance between this point and the centre of the lens (focal length) differs from one magnifying glass to another.

It is, of course, a known fact that sunlight carries a lot of energy and heats up the earthís surface everyday. When a magnifying glass is held over a piece of paper at a distance equal to its focal length, the sunís rays falling on the lens converge at a point that lies somewhere on the paper.

Thus, solar energy spread over the lensí surface area gets concentrated at one point. At this point, the paper gets heated up to a temperature higher than its ignition temperature which is 2330 C. Thatís when the paper starts burning.

In a way, the explanation for this phenomenon is analogous to how one explains the difference between force and pressure. A nail, hammered with a certain force, can be driven into a wall whereas an equally forceful punch wonít even cause a crack to appear on the wallís surface. The relevant factor is not the total force, but force per unit area, just as in the case of the paper, it is energy per unit area.

The question was sent by Sunil Kumar Pradhan from Jhargram

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