Let '2b' be the diameter of the circular portion and 'a' be the lengths of the other sides of the rectangle.
Total perimeter = 2a + 4b + pi b = K. ...(i)
Now, let the light transmission rate (per square metre) of the coloured glass be L and Q be the total amount of transmitted light.
Thus, Q is maximum and from Eqs. (i) and (ii), we get