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According to Lambert's law, the illumination on a surface is proportional to

According to Lambert's law, the illumination on a surface is proportional to 1/cos2θ.

The illumination on a surface is proportional to the cosine of the angle between the surface normal and the direction of the incident light. This law is also known as Lambert's cosine law.

E = I/(r2 cosθ)

where

E is the illuminance on the surface
I is the luminous intensity of the point source of light
r is the distance between the point source and the surface
θ is the angle between the surface normal and the direction of the incident light.