Question

Light is incident normally on a small plane surface. If the surface is rotated by an angle of 30° about the incident light, does the illuminance of the surface increase, decreases or remain same? Does your answer change if the light did not fall normally on the surface?

Answer 1

If the surface is rotated by 30⁰, the illuminance will decrease. This is because illuminance depends upon the cosine of the normal angle.
Yes, if the light does not fall normally on the surface initially, it may increase or decrease depending upon the former's angle. If the 30⁰ rotation brings the table closer to the normal of the surface, the illuminance will increase; otherwise, it will decrease.