The intensity of light from a source decreases with the distance x from the source, as x−2. In the following picture, the intensity of light from the source S at point A on a curtain is 8.1U where U is some unit. We then add a big mirror parallel to the curtain at point B. The distances AS and SB are equal, and the points A, S and B are in line perpendicular to the curtain and the mirror. What is the intensity of light reaching point A now?
[Figure: Light source S with point A on curtain at distance L, and mirror at point B also at distance L from S]
a) 9.0 U
b) 10.15 U
c) 10.8 U
d) 16.2 U