To assess the quality of a planed surface, a joiner looks along this surface, what is the law of optics he uses?

The carpenter, when assessing the quality of the processed surface, first of all evaluates the uniformity of its processing with a plane. If the entire surface is visible, then the processing quality is high. In this he is helped by the law of straightness of propagation of light, which states that light propagates in a straight line if the medium is transparent and homogeneous. The carpenter simply will not see any areas of the surface if the plane has not processed it flat.