How does using sunlight through a magnifying glass to start a fire work and how does it not violate the whole we can never create more energy then we put into it equation?
In: Biology
Magnifying glasses don’t create energy, they focus it.
Imagine I have an area that’s exactly 10 square inches and expose it to the sun. Each square inch gets, lets say, 2 units of sun energy. 2 units of sun energy is enough to make the area warm, but not doing anything.
Now I take my magnifying glass and focus it on the center square inch. The magnifying glass takes 1 sun energy from each other square and focuses it. So now the center square gets (2+9) units of sun energy for 11, and each other square is left with 1.
The square getting 11 units of sun energy gets very, very hot and the areas that only get 1 are actually *colder* than they would be without the magnifying glass. So energy is preserved and the one square catches on fire.
Physics rather than biology, the magnifying lens focuses the light so that no matter where on the lens it strikes it all comes together at a single point and with all that light energy in one location it can start a fire.
The energy is coming from the light source – the sun.
Every kind of matter that light can pass through without scattering, be it water, air, crystals, glass, whatever, bends the light. We know that as refraction. Light can pass through, say, a sheer piece of fabric, but from there, it’s not bent, but scattered, so we can’t see the stuff that’s not blocking the light on the other side. When light is refracted, it all gets bent the same way (mostly. There’s a tl;dr that could be added here). So a lens that bends light toward its center if held at exactly the right distance from an object, will cause all the light that would hit the area of the lens to hit a much smaller area. If regular sunlight can warm your body, it follows that more sunlight in the same area should heat that spot up more. If you use some dry grass or something, there can be enough heat to start burning it. And a fire is born.
Sorry if this is more ELI5 than you meant. Just wanted to make sure I covered the basics.
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