Two lenses A and B of focal lengths 7.5 cm and 20 cm respectively are placed at distances 15cm and 60cm from a point source of light ‘S’. The source ‘S’ is located on the common principal axis of the lenses. Aperture radii of the lenses A and B are 2cm and 10 cm respectively. When a screen is placed behind the lens B at a distance d cm from the lens B, a circular light spot of minimum radius r cm is formed on the screen.