The tremendous amount of heat that can be generated by bringing the light of the sun to a focal point through a lens is pretty amazing. Did you ever use a magnifying glass to burn ants when you were a kid? I can’t say that I ever did…but I did get to use one to burn leaves for a science class experiment.
This same concept can be applied on a larger scale for a variety of practical purposes, including cooking food.
You can make your own Fresnel lens for solar cooking by recycling old rear-projection television screens.
You can often find these TVs for free or cheap on Craigslist or at garage sales. The television does not have to work, but the screen has to be intact to be able to use the lens.
The lens will need to be mounted to some sort of frame with the rough side of the lens facing out. This is the side that collects and focuses the light to create heat.
As you can see in the video from TheGrowGeek below, the focal point of the light is pretty intense and the temperature gets very hot. By setting up your cooking surface near this focal point, you can easily generate enough heat to cook food.
Not all of the lenses from televisions are the same, as demonstrated in the video below from fencekid. Some have spot lenses, which focus the light to a single point, and some have linear lenses, which diffuse the light more, spreading it out over a line.
The spot lens creates such extreme heat at the focal point that it can start an almost instantaneous fire when wood or other flammable materials are placed under it.
The linear lens may not be as efficient at starting fires as the spot lens, but both lenses will work for cooking food.
While food can effectively be cooked by simply placing it underneath the lens, it is usually more efficient to cook on a grill or pan heated from below.
This can be done by bouncing the light from a spot lens is off a mirror, reflecting the heat up onto the bottom of a skillet or flat sheet of metal.