Do you like marshmallows cooked over a campfire? What if you don’t have a campfire, though? We’ll solve that problem by building our own food roaster – you can roast hot dogs, marshmallows, anything you want. And it’s battery-free, as this device is powered by the sun.
NOTE: This roaster is powerful enough to start fires! Use with adult supervision and a fire extinguisher handy.
If you’re roasting marshmallows, remember that they are white – the most reflective color you can get. If you coat your marshmallows with something darker (chocolate, perhaps?), your marshmallow will absorb the incoming light instead of reflecting it.
Here’s what you need to get:
- 7×10” page magnifier (Fresnel lens)
- Cardboard box, about a 10” cube
- Aluminum foil
- Hot glue, razor, scissors, tape
- Wooden skewers (BBQ-style)
- Chocolate, marshmallows, & graham crackers
Here’s what you do:
Download Student Worksheet & Exercises
How does it do that? The Fresnel lens is a lot like a magnifying glass. In Unit 9, we learned how convex lenses are thicker in the middle (you can feel it with your fingers). A Fresnel lens (first used in the 1800s to focus the beam in a lighthouse) has lots of ridges you can feel with your fingers. It’s basically a series of magnifying lenses stacked together in rings (like in a tree trunk) to magnify an image.
The best thing about Fresnel lenses is that they are lightweight, so they can be very large (which is why light houses used these designs). Fresnel lenses curve to keep the focus at the same point, no matter close your light source is.
The Fresnel lens in this project is focusing the incoming sunlight much more powerfully than a regular hand held magnifier. But focusing the light is only part of the story with your roaster. The other part is how your food cooks as the light hits it. If your food is light-colored, it’s going to cook slower than darker (or charred) food. Notice how the burnt spots on your food heat up more quickly!
Scientifically Dissecting a Marshmallow
Plants take in energy (from the sun), water, and carbon dioxide (which is carbon and oxygen) and create sugar, giving off the oxygen. In other words: carbon + water + energy = sugar
- In this experiment, we will reverse this equation, by roasting a marshmallow, which is mostly sugar.
- When you roast your marshmallow, first notice the black color. This is the carbon.
- Next notice the heat and light given off. These are two forms of energy.
- Finally, put the roasting marshmallow if a mason jar. Notice that condensation forms on the sides. This is the water.
So, by roasting the marshmallow, we showed: sugar = carbon + water + energy!
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You can do this experiment right in the snow, it doesn’t have to do with ambient temperature as much as focusing incoming energy from the sun down onto a marshmallow. In fact, one parent shared how they assumed they could leave it in the sun on a cold winter day and not stay and watch, and after 10 minutes they came back and were very surprised to find the cardboard on fire and melted plastic smoking, and it was happening right on the snow where they had placed it. So yes, go for it as long as the sun is out, and do NOT leave it alone!
We’ve just been introduced to this experiment and it is winter in Denver – snow all over and temp about 10-20 degrees F. Will it be worth trying to do this experiment right now? Should we wait for less cold conditions?
You can find all the review information from current mechanical engineering lessons by clicking on “Live Classes” from the top menu. Then scroll down to January and click the title “January – Mechanical Engineering”. Also, feel free to email [email protected] anytime you have questions.
You can purchase a fresnel lens for less than $10.00. Amazon has many to choose from. Just search “fresnel page magnifier”.
i couldnt find another general area to ask a question,
is there a review slides workbook available for Mechanical engineering?
How much would a fresnel lens cost? And where would I buy it?
This is a sick way to cook food! thx
You need the power of the sun in order for this to work. A light bulb won’t be bright enough. I recommend building this experiment, even if it is cold outside. As long as it is a sunny day, the oven will still get quite hot.
Can we use a lightbulb as it snows 7 months in Winnipeg and probably won’t work in the cold
Muhammad Khan, Age 13
Yes sunglasses are recommended when looking at bright objects. You can also stack a few pairs on if you need them to be darker. When we make solar telescopes, we use a number 14 welder’s plate or a piece of Baader film, but that’s only for looking at something as bright as the sun safely.
We had a lot of fun with this experiment today. I wonder — what (if anything) do you recommend for eye protection? When looking at the marshmallows to get the point of light focused correctly and make sure it stayed lined up, we were seeing green spots and our eyes were hurting a bit by the end! Are regular sunglasses protection enough? Thanks!
That is way cool
Followed the directions for the roaster but we had some cloud cover. We will try it again when the sun is more intense (noon). We did make the some-mores and that will serve as an incentive to try the experiment again.
We have had fun trying various experiments.
Thank you for wonderful hands-on introduction to real science!
I am glad your son enjoyed the project! And it sounds like you also had a fire safety lesson! I’ll post your comments to the website so other kids can learn from your experience also. Amazing how powerful little magnification can be, isn’t it? You can extend this lesson over to optics by sharing this piece with your son: Imagine if you looked through something with lenses at the sun… your eyeball would have had the same thing happen to it as your solar oven. That’s why you don’t look at the sun through anything with lenses.
Stay safe, and happy experimenting!
Aurora
Hi Aurora,
My son enjoyed building his solar oven today. However, he put it outside in the sun and the whole thing caught on fire. No harm done. I just thought you might want to know to warn others.
Thanks,
Elaine
Don’t forget that it does matter how far the lens is from the marshmallow… if you get it just right, then you’ll not only cook it but burn it to a point where it catches on fire (see previous comments). Enjoy!
i made it, but it was pretty slow, and only maxed out at about 135 degrees farenheit or so, and that was after about an hour. But then i did a marshmallow with a magnifying glass only, and after 5 mins i had managed to burn my initials on the marshmallow! (mmm my initials sue taste good…)
-Peter, Son of Natalie
We made our cooker and took it outside. We tilted it so the sun would hit the
marshmellow straight on. Forgot to look down on the ground and immediately burned a
little hole in the plastic outside chair! 🙂 After a while we left it inside the
house on the floor with the sun shining through the window. We went shopping and
left it to my husband to keep an eye on it. Apparently it started smoking and burned
some of the cardboard. Too bad we missed the excitement. Thanks so much. We had a
great time.
Glynns
Picture of our solar oven
Haley, Hannah, & Zach (& mom)
Sorry you had trouble! I had a marshmallow scorch after only 15 seconds. Glad you figured something out! 🙂
Didn’t work. Worked better without the box. 🙁
Wow! This is really cool!
Your marshmallow should cook in under a minute. Try adjusting the spacing between the marshmallow and the lens itself, the way you’d focus a hand-held magnifier.
Ok finally got a lens and so far it’s been 35 mins and the marshmallow is a little warm. Have tried directing the lens more to sun by propping up the box with rocks. Still no browning. Any suggestions on what we might of did wrong or is this typical?
Mine took about ten seconds.
If you’re having trouble getting it to work, don’t worry – science can be like that sometimes! But don’t give up – this is a great opportunity to learn how real life can be sometimes – keep trying! Here’s a hint – is your lens facing the sun? The lens is focusing the light down to a point, so it does matter where you place the marshmallow. It’s like adjusting the focal point when you’re trying to burn leaves with a handheld magnifying lens. You may want to play with the distance that the food is from your lens as well as aligning the lens with the sun.
We had our oven out for over a half hour in direct sun on an August day in PA. The marshmallow got a little gooey but never browned. The ripples on our lens were pointed out. Did we miss something or does it take that long?
Sure – you can cook anything you want with your roaster. Try different colored foods to see which cooks the best. If you use a larger food, you’ll be able to see where the energy gets focused down to and the area range of your cooker.
Hi,
I was wondering if I needed to cook anything larger than a marshmallow, would i need a larger window at the back? Also, can i put things inside the roaster?
Rgards,
Anand
Mine caught on fire! Oops…