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, decrease or remain same? Does your answer change if the light did not fall normally on the surface?
The illuminance through any surface is dependent upon cosine of the normal angle. So, if the any surface is rotated by 300 the illuminance decreases.
As the illuminance depends on the cosine of the normal angle. So, the illuminance changes when light incident at inclined angle. The illuminance increases, decreases or remains the same depending on the angle.