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a white boy squats on grass with a magnifying glass

Using magnifying glasses to focus sunlight and start a fire

Be careful with fire!

Be very careful with fire, because it can burn you, or burn down your house. If you’re not an adult, make sure an adult knows when you do this experiment and thinks you are safe.

Combustion reactions
Sunlight and photons
All our physics articles

Why does grass burn?

Grass, like all living things, is made out of hydrocarbon molecules. Oxygen is in the air around you. So if you put dry grass and oxygen together and get them hot enough, they’ll react with each other and release energy in the form of a flame.

What are hydrocarbons?
Heat and chemistry

Magnifying glasses and fire

On a sunny day, pull up a handful of dry grass and put it on your driveway or somewhere that won’t burn. Get a bucket of water and put it next to you. Use a magnifying glass (or a pair of glasses will work) to focus the sunlight on your grass, and wait for it to hear up and burst into flame. Get the focus as tight as you can, so the sunlight is all focused on just one small spot on the grass, like the head of a pin.

Put the fire out!

After you do get a fire, make sure to PUT THE FIRE OUT with the water before you turn your back on the fire or step away.

For further thought:

What do you think would happen if you tried to do this in space? How about if you put the grass under a glass cup and then focused the sunlight on it through the glass? What happens if you use pieces of metal wire instead of the grass? How about eggshells? Rocks? A piece of ham? What happens if you put sunscreen on the ham? Make predictions, and then see whether you are right.

Ancient Greek use of burning lenses
More about combustion reactions
A slower kind of reaction: rust

Bibliography and further reading about fire and science projects:

More about fire
Chemistry home