Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] fire starter from the sun



Aha, so there are two issues, the size of the lens, since the larger the lens the more energy collected, and the size of the image formed, since the smaller the size, the more concentrated the energy. There is also John's point that if the image is too small, there could be conduction problems.
I'm beginning to see the light!

cheers


Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Apr 16, 2006, at 11:24 AM, Brian Whatcott wrote:

At 08:00 AM 4/16/2006, you wrote:
I can understand why the diameter of the lens in important, the
larger the area, the more energy focussed, but I don't really
understand why the focal length would play such a large role. It
certainly is part of the f-stop formulation, but I don't see how it
could have a large impact on fire starting. Care to explain?

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.


In the Socratic manner:
It is possible to choose a lens which gives a small image of a scene,
or a large
image. Which sort of image would portray the image of the sun as smaller?

Supposing that the effective Solar energy density is 700 W/m^2
would a large diameter lens, or a small diameter lens pass more energy?

One would want a lens which passes more energy, and one which provides
the smaller image, you may have concluded.
What common lens specifiers does this conclusion lead one to advise?



Brian Whatcott Altus OK Eureka!


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l