There are a couple of different ideas going on
in a solar oven. The first, is using the foil
to collect more sunlight. The foil that you
use is intended to reflect more of the sunlight
into the box. The bigger you can make these
panels, the more light you can get into your box,
and the hotter it will get. This is the same
basic idea as shining light through a magnifying
glass. As you probably have done at some point,
if you use a magnifying glass to focus sunlight to
a single spot, you can get that smaller area much
hotter. Using the foil is the same idea.
Now, the second idea of a solar oven is
having something absorb the light. Any
object you put in there will absorb some light, so
having increased the amount of light entering the
box, you can make it get hotter. Some types of
ovens will put dark (black) objects into the oven,
because dark objects absorb the most light and
don't reflect as much back. This means that
dark objects will get hotter than light objects.
In these types of ovens, sometimes food might be
set on top of a dark object, or even put inside of
it, to try and get it to a higher temperature.
Foil is reflective (being metal), and light
is energy.In trapping the food inside a region
where only it can absorb the light, you are
focusing the light of the sun on the food
over a much greater area (i.e. the box) than the
Basically, it's focused using mirrors. The idea is
the same as using a magnifying glass - you're
focusing energy on a small area. Archimedes
developed mirrors as a weapon that could set ships
on fire using this principle.
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