Before you measure anything: know what you’re measuring from.
01Find the point where the ray meets the surface, and draw the normal there.The normal is a construction line you put in yourself. It isn’t printed on the apparatus, and no angle in the question can be measured until it exists.
02Put the protractor’s centre mark exactly on that point.The centre is the pivot the whole scale is measured about. A centre a few millimetres off gives an angle about the wrong point, and the error grows with how short your ray is.
03Turn the protractor until its baseline lies along the NORMAL.This is the step the straight edge fights you on: the surface is the obvious line to lay it against. Resist it, or commit to reading from the surface and subtracting from 90° every single time.
04Check that the ray falls inside the arc; if it does not, turn the protractor over.The arc covers 180° on one side of the baseline. Incident and reflected rays sit on opposite sides of the normal, so one placement won’t read both.
05Read where the ray crosses the scale, choosing the ring that starts at zero on YOUR baseline.Both rings of numbers are printed against the same marks. The one that reads zero along your baseline is the one that’s measuring your angle; the other reads 180° minus it.
06Write the angle to the nearest half degree.The divisions are whole degrees, so half of one is the allowance. Neither round to the nearest ten nor claim a tenth the scale can’t show.