Reflective Solar Cooker
This activity will take you 30 minutes to build and
about 45 more minutes to complete, depending on
how many experimental trials you conduct.
This reflective solar cooker uses the Sun’s energy to
cook marshmallows. The target cooking area is the
space where the light concentration is greatest.
Never look directly at the Sun! It could damage
your eyes. Don’t allow the cooker to reflect sunlight
into your eyes.
ACTIVITY
You will need these items to complete this activity:
• shoebox
• aluminum foil
• string
• tape
Preparation
1. Cut out slots of equal length down the short sides
of the shoebox opposite each other. Draw a scale, beginning
with zero at the top, along each slot. Then
cut diagonal slits at the corners of the box for the
string.
2. Cut a manila folder in half along the fold. Place
one half inside the shoebox, so that the folder bows
into a curved, half-pipe shape resting on the bottom
of the box. Fasten with tape in this shape to the box.
3. Lay a sheet of aluminum foil, shiny-side up, along
the curved folder. Tape it to the box, fitting it to the
folder shape.
4. Cut two 20-inch lengths of string. Knot each at
one end. Floss the knotted ends into slits A and B.
Drape the string inside the box, and insert the other
end into slits C and D.
5. Discuss the term “cook” with the class.
Experiment
1. Place one white marshmallow onto a straw near
the end, and a colored marshmallow (or more, if you
have several colors) on a second straw.
2. Snip a slit at one end of the second straw, and then
join the slit end to the other straw. Space the two
marshmallows an inch or two apart from each other.
3. Lay the straw into the slot so that the marshmallows
are near the center of the box. The straws
should rest on the string at both ends.
4. Pull on both strings to bring the straw to the first
level from the bottom.
5. Direct the box toward the Sun; prop it up. Allow
the marshmallows to cook for a specified time (e.g., 4
minutes).
6. Repeat with another set of marshmallows at a different
height for the same length of time.
7. Option. After discussion, when the best height is
discussed, repeat the experiment with different cooking
times.
Analysis
1. Why is the shiny surface curved? Would this work
if it were straight?
2. Did the color of the marshmallow make a difference?
Why?
3. Did the height of the straws make a difference? Why?
4. Did the student definitions of “cook” change?
Answers
1. It is curved to focus the sunlight. A straight surface
will reflect but not focus light.
2. The darker color marshmallows should cook faster,
since white reflects rather than absorbs energy. (This
is the reason you are cooler when you wear white
clothes in the sunlight than when you wear dark colors.)
3. When the straws bring the marshmallows closest to
where the Sun’s energy is concentrated, the marshmallows
will cook fastest. Imagine that the reflecting
surface is part of a circular pipe; depending on the
shape of your box, it may not be perfectly circular.
The focus is one half of the radius of this pipe.
Hint: Students may notice the marshmallows are
brightest when they are near the focus.
4. Most students expect “campfire” results before the
experiment.
National Science Education Standards
Earth Science (5-8): The Sun is the major source of
energy for phenomena on Earth’s surface.
Science and Technology (5-8): Students should develop
abilities of technological design.
0
1
2
3
4
5
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9
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Reflecting surface =
aluminum foil-lined
manila folder
Slit A
Straw
String
Slit C
Slit D
Marshmallow
Slit B
For Spanish version and to comment visit http://www.as.utexas.edu/mcdonald/scope/scope.html ©2000 The University of Texas McDonald Observatory
• straws
• manila folder
• marshmallows (white and
chocolate, or other color)
