A point source of light S is placed at distance 10 cm from the pole of a convex mirror of local length 10 cm. The intensity of light at the center of screen in the absence of mirror is 49Wm−2 . Find the intensity of light on screen in the presence of mirror.

Here, Intensity without mirror is   I0=Po4π0.22where P0 is power of source. The intensity at the side of mirror is  I=P04π0.12=I04π0.224π0.12=4I0The power of incident on mirror is Pm=πd24×4I0=πd2I0Here, d is aperture of mirror, ∵u=−10cm,f=+10cm ∵1v+1u=1f ∴v=5 cmFrom similar triangles, Dd=355 ∴D=7dIntensity due to reflected light on the screen is I1=4Pmπ7d2=4πd2I0π49d2=4I049Total intensity =  I0+4I049=53I049                               =53×4949=53 Wm−2