Naked Science Forum

General Science => General Science => Topic started by: Patti on 16/02/2009 16:30:03

Title: Can you start a fire with a magnifying glass but without the Sun?
Post by: Patti on 16/02/2009 16:30:03

Patti asked the Naked Scientists:
   
Can you start a fire with a magnifying glass and a light source that's not the sun?

What do you think?

Title: Can you start a fire with a magnifying glass but without the Sun?
Post by: swansont on 16/02/2009 16:41:25

Sure.  You can use a laser, and it will actually be a little easier in at least one way; being monochromatic, you don't have to worry about chromatic aberration of the lens, which makes it so you can't focus the different wavelengths at the same point.  I've melted plastic (and burned myself) with less than 1W of laser light.

Any source of light that's approaching 0.1W/cm^2 has as much energy as sunlight in the same area.

Title: Can you start a fire with a magnifying glass but without the Sun?
Post by: Andrew K Fletcher on 16/02/2009 16:55:00

Maybe if you rub it very hard and very fast on a log you might get a flame.

Title: Can you start a fire with a magnifying glass but without the Sun?
Post by: lyner on 16/02/2009 18:11:11

Actually, once the 'hot spot' is reasonably small, it can still cause a fire so CA may not be much of a problem. The most important thing is the total Watts falling on a smallish region. Yes, the peak temperature of the spot counts but, to start an actual fire, there is also a total power requirement - i.e. diameter of gatherer.

As for boiling water and solar ovens - the reflector / lens can be pretty low grade because the target is so big. - I used a 0.5m satellite dish with kitchen foil stuck to it and could boil a small amount of water almost as fast as a bunsen burner.