If surface is perpendicular to incident light rays, it is illuminated more brightly.
The surfaces which are oblique to the light source are illuminated less brightly.
cos I = N. L
where, I = Angle of incidence
N= Vector of length 1 in direction normal to surface.
L= Vector of length 1 pointing towards light source.
Il dif= ka. Ia + kd. Il (cos I)
Where, Ildif = Shade of visible surface of an object
ka. Ia = Intensity of light coming from visible surface due to diffuse illumination