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.
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 t rapping 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 food itself.
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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