Physics - Geometrical Optics Question with Solution | TestHub

A converging lens of focal length $40\sqrt{2}\mathrm{cm}$ and diameter 10 cm is kept normal to sunlight and is used to produce a focused image of the sun on a black $(a=e=1)$ disc of same size as the image. Neglect the temperature of the surrounding. The temperature of the disc in steady state is $(n\times 100)\mathrm{K}$. What is the value of n ? Take temperature of sun as 6000 K . The disc is perfectly conducting and radiating from both the sides.