8.22 A solar furnace uses a perfectly reflecting concave mirror of aper- ture 0.2 m and focal length 2 m. At the position of the image of the sun is a perfectly absorbing sphere 2 mm in diameter made of a metal with a specific heat capacity of 400 J Kkg and density 8 mg mm. Find (a) the initial rate of rise in temperature of the sphere, (b) the final temperature attainable. [Angle subtended at the earth by the sun = 9 10 rad. Temperature of the sun = 6000 K.]