Magnifying glass

[rabsyboy]

I had a big experiment day with magnifying lens a few weeks ago when it was really sunny.
I used scrunched up newspaper as a test

A Magnifying lens about 2 inches in diameter I bought specifically to light fires with from BCM wouldn't work - the concentrated beam burnt little holes right through the paper, and I got alot of ash, but no fire - even when I broadened the beam out alot so it was spread out more.

A small watch makers magnifying lens wasn't big enough to catch enough light.
No good at all.

A plastic lens I found at work, and appropriated once again burnt through the paper and created lots of ash but no fire.

I bought some fresnel lens, and these seem to be the best ones. You don't need alot of sun (though it doesn't work if its cloudy obviously), and they take up no room at all hardly, and have no weight.

Plus the credit card sized ones are only £1.25 on ebay.

The big A4 ones are excellent too - but too big to carry about in your pocket.
I think in a country where sun light is weak for the most part size DOES matter.


I think my neighbours think I'm a bit strange.....

 

[clcuckow]

Well, I've learned something today, about the misinformation !

It was not misinformation as the magnification is important as without it we could not focus the energy and we would also get fried every time we walked past a window on a sunny day It is just that lens size is more important (just look at both newtonian and refractor telescopes.) of the two.

 

[fredcraft]

Thanks for the clarification cuckow ! Optics is really a fascinating field.

 

[mace242]

I was planning on doing this tonight - we've had a few days of great sun. And now... it's chucking it down

 

[rich59]

You know size shouldn't matter! But in reality is certainly does. I have some thoughts on why size matters and it suggests what might be done to solve the problem.

I spent some time trying small lenses - particularly I wanted to get a water drop lens to work. I was thinking that what ever the lens size what it is doing is achieving a small image of the sun on your tinder. The amount of energy you get per square nanometer (smaller than a millimeter) is probably the same whatever the lens size as long as it is a good lens. The problem with small lenses is, I think, that because the hot spot is so small the amount of loss of heat from the hot spot is relatively much larger. Therefore, if one could insulate the fried spot on the tinder really well then it should create the energy needed to start the material glowing. So much for theory. The elusive drop of water fire starter still elludes me