Why must rays be shone in the centre of a flat surface? Describe the difficulty that's results if this is not done.
4 answers
Your question must be in some specific context you did not share. Rays can be shown on any surface, curved, or flat, in the center or not.
We did an experiment of Snell's law of refraction and we shined a light through the centre of semicircular prism on a flat surface to measure the angle of refraction. That was one of our questions I didn't understand necessarily why.
In an experiment like that, the light beam (which I assume was a narrow laser beam) must strike midway between the edges of semicircular prism, so that it is refracted along a radius of the semicircle and strikes the semicircular surface at a right angle. In that matter, it is not refracted a second time, so the angle of refraction at the flat surface can easily be measured.
The word "matter" in my answer above should be "manner"