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

There will be a decrease in the illuminance when there is a rotation of the surface by 30 degrees.

Explanation:

• This is due to the reason that the illuminance is dependent on the normal angle’s cosine.

• So, yes, if on the surface, the light does not normally fall, then it may decrease or increase depending on the angle of the former.

• If the rotation of the table to 30 degree brings closer to the surface’s normal, then there will be an increase in the illuminance. Else, there will be a decrease.