Marshmallow Ignition

This is a science demo for discussing concepts like albedo, heat transfer, and how telescopes work. It relies on setting up a parabolic mirror, much like you would find as the primary mirror in a newtonian telescope, and pointing it back towards the Sun. By placing objects in the focal plane of the mirror, you can heat them enough to get them to burn, much like focusing solar rays with a magnifying glass. We place a marshmallow on a stick at the focal plane and observe that it does not ignite, due to the high albedo (i.e., reflectance) of the marshmallow--it just gets very very bright. However, when we dip the marshmallow in cocoa powder, it drops its albedo, causing more of the incident radiation to be absorbed than reflected, which leads to the marshmallow igniting almost instantly when placed at the focal plane of the mirror. This demo is very effective for teaching these concepts and it has a high profile due to smoke and smell of burning sugar around environment, so it is often a science expo favorite. Great care must be taken to maintain safety with this science demo, as permanent damage and injury are a very real possibility.

Materials Required

• Parabolic mirror with mount for tripod
• Tripod
• Fabric for safely covering mirror
• Jumbo-sized marshmallows
• Small box of cocoa powder
• Little sticks for roasting marshmallows
• Dark sunglasses
• White board with dry-erase marker for drawing optical diagram
• H-alpha telescope with mount, battery, and cord (optional)
• 4" dobsonian with solar filter or sun funnel (optional)

Setup Procedures

1. Find a location in direct sunlight to set up the tripod. You will want to ensure a spot that has direct sunlight over the course of the period you'll be running the demo.
2. Set up tripod at maximum height, and use top crank to increase height even more. Hang something moderately heavy from the hook in middle of tripod to help stabilize it.
3. While ensuring that the parabolic mirror is covered with fabric to prevent off-axis light burning something, attach the back of the mirror to the tripod.
4. Aim mirror back towards Sun, ensuring that the Sun is reasonably high in the sky so that it is impossible for anyone to inadvertently step into the focal path. Remove fabric covering mirror.
5. Put on strong sunglasses to help minimize glare when placing marshmallow in focal plane of mirror.

Procedures for Demo

1. Once you have an audience, describe how this curved mirror (parabolic) is similar to the mirrors inside of telescopes and are used for focusing a larger aperture of light to a smaller region.
2. Most of the time astronomers want to do this in order to collect lots of light from faint things (stars, galaxies, etc.) so that we can see them better.
3. You can show them a small dobsonian telescope or an optical diagram to further highlight how this works.
4. Further explain that in this case, we're going to do it focus light from a very bright object, the Sun.
5. Skewer a marshmallow with one of the sticks. Ask audience members what they think will happen when you place the marshmallow in the focal plane. Discuss.
6. Hold marshmallow in center of mirror near the glass, then slowly move it outward toward the focal plane. Focal distance for our mirror is about 8" away from mirror.
7. You'll know when you've reached focal distance roughly by how bright the marshmallow appears in reflected light! Discuss what is happening. Introduce topic of albedo (reflectance).
8. Talk about different objects in the sky and what their albedos are. Discuss how the marshmallow has a high albedo and reflects most of the incident light.
9. Now lightly dip the marshmallow in cocoa powder so it's roughly coated with cocoa. This will decrease the albedo and make more radiation get absorbed by the marshmallow.
10. Ask your audience to predict what will happen when you place it at the focal plane and why?
11. Proceed with holding the marshmallow at the focal plane as before. It should ignite very quickly showing signs of smoke and sometimes a small flame.
12. After you've burned it sufficiently, feel free to hand it out to audience members as they desire, but note that it is hot, since it was just on fire.
13. Some audience members will want just a marshmallow, so that's OK to give them without burning first if so desired.
14. Be mindful of cocoa powder getting on the mirror. It will burn the mirror, so please blow or wipe excess cocoa off of the mirror as it needs it.

Notes on Safety

1. Only Caltech Astro Outreach volunteers should be operating this demo. Due to the safety concerns, please do not allow anyone else to ignite marshmallows.
2. Please be mindful of anything that might get in the optical path of the sun coming off the mirror. Sometimes people want to put their hand at the focal plane--do not let them do this. A guy inadvertently lit his umbrella on fire getting too close.
3. As the sun changes its position over the course of the day, you'll need to move the mirror as well. Failing to do so can change the focal position of the mirror into new locations.
4. Please wear dark sunglasses when operating this demo. The marshmallow gets very bright in the focal plane, and you might cause minor damage to your eye by looking at it for too long.
5. Do not burn anything other than marshmallows in the focal plane. We've had people want us to melt crayons or burn paper--too many problems, so just stick with marshmallows.
6. All told, just be careful with this demo. It's super fun, but it must be handled with care to avoid injury. This is one that we need to take seriously.

Link back to home page.