To assess the quality of a plane cut, a carpenter looks along that surface. What optics law does he use?

In this case, the carpenter uses the law of rectilinear propagation of light: “Light in a transparent and homogeneous medium propagates in a straight line.” If the workpiece has a flat surface, then the carpenter, looking at it along, will see the entire plane of the workpiece.

By the way, this law is applied when determining the distance to selected objects.

This law explains the formation of shadow and penumbra, solar and lunar eclipses.