Optically, a parabola is a shape that allows to focus sunlight in a single point.
Mathematically, it is an equation of the type y = ax2
With a few tools and a flexible mirror, a parabola can serve as a solar oven!
calculator or excel, ruler and pencil, saw,
wood, adhesive mirror, semi-rigid plastic sheet, metallic tube, black paint, transparent plastic
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Step 1: Calculate the Size and Shape for Your Parabolic Mirror
Define the shape of the parabolic reflector (length, depth) according to your supplies. The shape of the parabola can be more or less open according to the parameter a.
Find the equation that correspond to this by iteration
for example, in centimeters, y=0.004 x2 makes a parabolic reflector of 200 cm long and 40 cm deep
Use excel or your preferred calculator and calculate y for various x
Step 2: Draw the Parabola on a Planck
Using the table you've just calculated, draw the parabola on a wooden board to make the parabolic support of the reflector.
Cut two identical pieces of wood usin a jigsaw for example, they should be wide enough to stay strong (e.g. 5 cm)
Then screw them together apart using a spacer of the width of your adhesive mirror (usually 50 cm)
Step 3: Stick the Mirror to the Plastic Sheet and Fix It to the Parabolic Wood Piece
Take a semi-rigid plastic and stick an adhesive mirro to it, you can also use mirrored plastic (but it's usually more expensive).
Then fix it to the wood structure you made. You have now a parabolic reflector.
Step 4: Build the Legs of the Reflector and the Support for the Tube
The reflector now needs some legs to be oriented towards the sun, you can use wood or metal, it should be stable to resist some wind.
The reflector will concentrate light on a line.
Paint a metallic tube in black and use it as a concentrator.
Build something to hold the tube, the distance between the tube and the reflector should equal the focal length of the chosen parabola.
a = 1 / 4f
f = 1 / 4a
in our case with a=0.004 the focal distance is 62 cm.
Step 5: Prepare Your Food, Turn the Reflector Toward the Sun and Enjoy !
Wait for a clear sky day, turn the reflector towards the sun and check that a line of concentrated light is formed on the black tube.
The tube will warm up quickly, you can place inside the food you want to cook. Salmon in papillote, in aluminium foil with some vegetables, will cook in about 45 minutes.
For best results (and to avoid to burn the tube holder), you should realign the reflector towards the sun every 15 min.
Bravo ! Now you are able to cook with the power of the sun ... and mathematics !
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