Does a magnifying glass' magnification level correspond to it's burn power?

Question

I'm wondering what kind of magnification glass I should get if I want to create the most heat with magnified sunlight. How much does the magnification power, ie 2x, 4x, 10x correspond to the ease with which it will burn things. How much does it depend on the SIZE of the lense? Also, any recommendations to inexpensive suppliers would be appreciated.

Answer 1

DUH. That's like, the most basic science knowledge that's even ever been PROVED.

Answer 2

When it comes to starting fires, the purpose of a magnifying glass is to gather and focus light. When a material absorbs part of that light, much of the energy is transformed to thermal energy, heating the material. Because this heat comes directly from light, and specifically, only the light gathered by the glass, the "hottest" magnifying glass would be the one that gathers and transmits the most light.

Gathering light is a simple matter. It is proportional to the cross-sectional area of the magnifying glass, or, to the square of the diameter of the magnifying glass. Hence, a larger magnifying glass would provide a higher "burn power."

On the other hand, transmitting light is somewhat out of your control. The quality of the glass used in simple magnifying glasses is undoubtedly low, so the glass itself will most likely reflect and absorb some of the light. A more expensive glass specifically designed for maximum transmission of light would provide a higher "burn power."

The magnifying power of the glass actually has very little to do with its "burn power." Magnification has to do with the focal length of the lens, which, in turn, has to do with the curvature of the lens. It simply controls how high above the combustible material you'll need to hold the lens. The intervening air will absorb only a negligible amount of energy from the focused light, so this aspect of the glass can be ignored.

In short: use the largest magnifying glass you can. Larger apertures gather more light; hence astronomers spend a great deal of money on large lenses and mirrors for their telescopes. Not to burn holes in their eyes, but to gather the most faint starlight.

Answer 3

Aperture is key, as the 2nd responder points out.  Following that advice, you can get the most "bang for the buck" in this application by using a Fresnel (pronounce Freh-nel' ) lens rather than a traditional spherical lens.  Fresnel lenses have huge apertures compared to spherical lenses, and are much cheaper. Most solar cookers use them.  Here is but one example:

http://scientificsonline.com/product.asp...