Why does paper burn under a convex lens aimed at Sunlight? 

3 Answers

Samuel Chiltern Profile
Samuel Chiltern answered
A convex lens can be used to focus the parallel rays of sunlight passing through it, into a singular point.

If this focal point is aimed accurately onto a piece of paper, then you will see a small image of the sun at the focal point, and this is where the paper will begin to burn.

How Does a Convex Lens Make Fire?
Any kind of combustible material (such as paper) will catch fire if it is exposed to enough heat, provided there is a ready source of oxygen available. By using a convex lens to aim the sunlight, you are focusing the heat energy into a small point, which makes the heat much more intense.

Convex lenses are used in magnifying glasses, so any magnifying glass will allow you to conduct this experiment.

Larger lenses 'collect' more of the sun's rays and so will be more powerful. Lenses with a higher magnification value will allow you to focus light onto a smaller point, which again increases the power.

How Can I Start a Fire Using a Magnifying Glass?
  1. Make sure that the sun is shining directly through the magnifying glass in a straight line onto the paper. Never look at the sun through a magnifying glass - you could be blinded for life
  2. Tilt the glass, until the focused light becomes a perfect circle - not an ellipsis
  3. Change the distance between the magnifying glass and the sun, until you find the 'sweet spot' where the focused light is as small and as bright as possible, whilst keeping it circular. Don't stare at this light for too long, because even reflected sunlight could damage your eyes
  4. Hold the glass steady, maintaining the focus. Move it around a little to heat up a larger area, then it should catch fire
Nic Turner Profile
Nic Turner answered
A convex lens is often called a converging mirror, because it is able to converge rays of light to a point. It converges sunlight to a point, which in turn ignites the paper.
Anonymous Profile
Anonymous answered

When any light is focused on a particular point it always destroys or leaves a mark on the object, specially if its sunlight -the most powerful light

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