A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39×10⁹ m and its mean distance from the earth is 1.5×10¹¹ m. What is the diameter of the sun's image on the paper?

Question

Accepted Answer

(1) 9.2×10⁻⁴ m. Given: f=10 cm=0.10 m, the sun's diameter O=1.39×10⁹ m and its distance u=1.5×10¹¹ m. Idea: the sun is effectively at infinity, so its image forms in the focal plane and v=f. Object and image subtend the same angle at the lens, so I/O=v/u. I=1.39×10⁹×(0.10)/(1.5×10¹¹)=(1.39×10⁸)/(1.5×10¹¹) I=9.27×10⁻⁴ m, printed in the options as 9.2×10⁻⁴ m.

A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39×10⁹ m and its mean distance from the earth is 1.5×10¹¹ m. What is the diameter of the sun's image on the paper?

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Why the other options are wrong