Question

Two magnifying glasses are for sale at a store. Magnifying glass 1 has a 4-in diameter with ling focal length, and glass 2 has 1 -in diameter with a short focal length. a) Which mannifying glass should you purchase if you wish to examine tiny insects? b) Which glass should you purchase if you wish to start a campfire using sunlight?

Answer 1

a. While examining tiny insects, one would want to have higher magnifying power so that the tiny insects can be seen as magnified as possible. Power is inversely proportional to focal length. Hence the lower focal length glass is better for this surface. Having a lower diameter is not as important as the magnifying power is.

b. If we want to start a campfire using sunlight, we are going to use the magnifying glass for a very different purpose. Here we are not even interested to see the image. Rather it is the image that is concentrated onto a small area. Here the diameter of the lens is an important factor. A wider lens will allow more sunlight to get focussed. Doubling the diameter amounts to making the area four times. Hence, the amount of light energy will also be four times. This would lead to the ignition of fire in lesser time. The effect of longer wavelegth will be that it has to be kept at a longer distance from the burning material which is a secondary aspect here.