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I have seen videos where a fire was started using a water bottle filled with water. I guess the bottle acts like a convex lens that converges light to a focal point to start a fire. But why does water have to be added?

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It's the water itself that forms the lens. Lenses work via refraction. The refractive index of water is about 1.333, which is different from the refractive index of air (about 1.0), so rays of light bend at the junction of the air and the water.

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