Measure the distance from the card corner or rule mark to the wire, if any. Consider possible errors. Move it away and line it up again. Since the days of Ibn al-Haytham, many philosophers and scientists have discussed the foundations of the scientific method of inquiry. Most scientists accept Karl Popper s view that science can never prove a theory.
Experiments can only disprove an incorrect theory.
A theory is never proved, but becomes accepted when it has been tested often and never disproved. Our experiment is evidence in favor of the rectilinear propagation of light. We have not proved it, but in principle we have done as well as any scientist ever can. Experiments just like ours, in which nothing observed contradicts any accepted theory, can be significant and important in science. Sometimes, when better experimental techniques become available, scientists repeat an old test of an accepted theory to make sure it is true.