When I put a lens along the the sun it convert the sun energy to heat and maybe fire the substance beyond the lens. How could I determine the heat produced and what is the formula?
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If one assumes a lens that perfectly focuses all incident light into the focal point, the light power at that point equals the power that hits the surface of the lens.
The solar constant gives the power the sun transmits onto one square meter of the earth, and is approximately $1.36\,\mathrm{\frac{kW}{m^2}}$. If we assume that $50\%$ of that power actually reaches the Earth's surface (reference, although I am not sure how reliable it is), the power available per square meter is $p = 680 \, \mathrm{\frac{W}{m^2}}$.
Assuming our perfect lens is circular and has a radius of $R=0.1\,\mathrm{m}$, it has an area of $A=R^2\pi\approx0.03\,\mathrm{m^2}$, so that the total power transmitted to the lenses focal point is $P=Ap\approx21\,\mathrm{W}$.
However, not all of that power would be converted into heat as you shine it onto some object, since that object will reflect part of the incident radiation instead of absorbing it. What fraction of the incident power actually heats your object up depends on the material.
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