In bright sunlight, we can use a magnifying glass to start a fire on some dry leaves. Explain how this is poss

Question

I would like answers to be secondary school standard ( 13-16 yrs old)

Answer 1

'A burning-glass is a large convex lens, which can focus the sun's rays on a small area and so ignite materials. Used in 18th century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis, the burning-glass was a useful contrivance in the days before electrical ignition was easily achieved.'

A magnifying glass is in fact a convex (or a biconvex if you like)lens as mentioned in the para above from encyclopedia.

When it is held between the shining Sun and any combustible material in such a way that the Sun rays, after passing through the lens,are refracted or bent and focusd on that material.

The heat generated is enough to ignite it.

In fact one is creating the image of the Sun on that material along with its heat.

Perfect focusing is the Key factor in the entire exercise.

Kindly click on the link below to see the diagram.--

http://en.wikipedia.org/wiki/Biconvex_lens#Types_of_lenses

It is for the same reason it is not good to look at the sun without goggles for a long time. Why ?

We have a convex lens in the eye. It focuses everything on the Retina at the back of the eye . And because of the perfect image , we see everything perfectly.

http://en.wikipedia.org/wiki/Eye_lens

If one looks at the sun with naked eyes , the same lens will cast the perfect image of the Sun on the retina and literally cause burns in it, damaging it permanently.

Answer 2

The magnifying glass you talk of bends light beams or more particularly sunlight towards a point on a line through its horizontal axis, called the focus or focal point. When the light beam is strong enough, as in very bright sunlight, the concentrated light is intense enough to ignite any flammable substance on which it falls, for example dry leaves, cigarette, paper and worst of all, petroleum products which can be quite harzardous. So please, teach the young ones mentioned not to fool around with this device.

Answer 3

To make it really simple: the sun gives off heat in the form of UV rays. All the magnifying glass does is concentrates the rays in one small spot, thus greatly increasing the heat in that one small spot. the larger the diameter of the magnifying glass, and the smaller the spot you can focus it on, the more heat you get.

Compare it to the heat from one match as compared to whole large book of them being ignited at once, all together. more heat and a bigger flame results.

Answer 4

The magnifying glass is able to refract the light waves, allowing them to bend differently to how they were dispersed by the sun. The shape of the magnifying glass causes the light waves to refract and bend in such a way that the waves are able to be focussed on a particular point. This focussing of light waves causes the heat from the light energy to also be focussed in one spot. It is this heat that causes fire, especially on dry leaves as these are easy to set fire to and require little heat energy.

Answer 5

Light bends when it enters or leaves glass. Magnifying glasses are constructed so that parallel light entering converges onto a single point when leaving on the other side. The point where the light is concentrated gets hot.