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Why Does a Magnifying Glass Make Fire?
Certain physical laws make it possible to light a fire using a magnifying glass or other lens.
Light Energy
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Light emitted by the sun is made up of photons, tiny packets of electromagnetic energy. Photons behave both as particles and waves. This quality is important because the nature of photons means that they are refracted when they pass through different media. Photons passing through water change their speed, causing a ray of light to appear at a different angle above the water than below the water's surface.
Magnification
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The lens of a magnifying glass is curved to refract light particles in such a way that the eye perceives a larger image. This works only at certain distances, and when a magnifying glass is used at different distances it may flip the image upside down and make it look smaller, instead of larger. The important point to note is that a magnifying glass causes rays of light to change angle as they pass through the lens and meet at a specific point, focusing the energy of the photons.
Making Fire
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A combustible material, when exposed to heat and a source of oxygen, produces fire. Holding a magnifying glass so that the rays of light passing through the lens meet on a dry, flammable item, such as a piece of paper, concentrates the energy of the photons in a single spot and produces greater heat than the heat we feel on our skin from the sun. With this heat, it is not difficult to produce a flame. However, if the magnifying glass is held at the wrong angle or distance from the paper, the heat may not be great enough to light the paper on fire.
Potential
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Because it is possible to light a fire using a lens, a magnifying glass is a useful tool to carry when camping or traveling. In the presence of sunlight, it is a fairly reliable method, and unlike matches or a lighter, will not be damaged by moisture.
Other Lenses
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If a magnifying glass is not available, the lens of binoculars, eyeglasses, or another curved piece of glass may also work.
eHow Article: Why Does a Magnifying Glass Make Fire?
