Focus, kids!

The Primaria kids are learning about solar ovens; they fried an egg
yesterday and they baked cookies today.  I brought in a neat piece of
equipment to complement that: a parabolic mirror about 24" in
diameter.  If you point it at the Sun and put anything flammable at
the focus, it will burst into flame in about one second.

I also brought a larger flat mirror and started by having the kids try
to figure out how to set a piece of paper on fire.  (I knew they
wouldn't actually be able to do it without guidance, so there were no
safety issues at this stage.)  After watching what they did, we
discussed some very basic physics:

--you can't have too many people crowding around the mirror, just as you
  wouldn't put the mirror under a tree

--you want to point the mirror at the Sun

So I pointed the larger flat mirror at the Sun and had them try again.
When they couldn't, I pointed out how the "small" mirror was curved
and explained why that was important: light hitting the edge of the
mirror will be bounced toward a point above the center, and light
hitting the center will also be be bounced toward a point above the
center.  All the light meets at one point! 

With that in mind, I had the kids form a line and I helped them ignite
paper one by one for safety reasons (most of them were also too scared
to try by themselves).  I showed how putting the paper directly on the
mirror was no different than just putting the paper in regular
sunlight. As we lifted the paper above the surface of the mirror we
could see the light start to focus down on a smaller area of the
paper.  At just the right distance from the mirror, all the light is
focused on such a small area of the paper that it starts to smoke!

This one-by-one part of the activity took most of the time.  There was
little or no time for many of the add-ons I had imagined, such as
connecting with the idea of a magnifying glass, discussing how to
store solar energy so you can use it at night, relating to the
temperatures of different planets (concentrating the light mimics the
effect of being closer to the Sun), and having them draw how it works.
With some of the groups, I had brief discussions of some of these
ideas, but we definitely could have used more than the allotted 20
minutes.  I especially wish we could have done the drawings...it would
be really interesting to see what the kids would come up with.

I wore pretty dark sunglasses for this activity.  At one point I took
my sunglasses off to lend them to a child, and I was just about
blinded...I did have a blind spot for about 5 minutes afterward.  So I
would recommend bringing sunglasses for the kids too.

You don't necessarily need a mirror as large as the one I was able to borrow.  I've seen survival shows where people used parabolic mirrors as small as the ones in flashlights to start fires.  It just requires more patience, and more fine motor skills to hit that small focus.

If you want to add a little more physics, note that the Sun provides over one kilowatt per square meter (the "solar constant" is 1.361 kW/m2, but the atmosphere takes a bit of that).  Kids in the upper elementary grades should be able to figure out how many watts their mirror collects and compare that with the power of their microwave oven, the energy consumption of a typical house, etc.