Foucault Test

The point of the knife-edge could cross the light rays in one of three places. Right where they converge, in front or behind. Each produces a characteristic shadow pattern seen on the mirror. If the mirror darkens from the right, the knife-edge is in front of the point of convergence. If it darkens from the left, the knife-edge is behind this point. The results shown here are for a perfectly spherical mirror. If the mirror deviates from a spheroid, several different shadows occur at once. This is because some rays converge in front of the radius of curvature and some behind. The individual shadows indicate where slight indentations or bumps are.

Right at the Center of Curvature

In front of the Center of Curvature

Behind the Center of Curvature