Use left/right arrow to rotate surface; up/down arrow to rotate approach angle. B = 2 * S - A where B=bounce angle, S=surface angle, A=approach direction. https://scratch.mit.edu/projects/597917425/ is a dem
This crops up on the forum from time to time. Basic bouncing off a flat surface can be calculated as shown. If something approaches at angle A and hits the surface which has angle S it will bounce off at angle B. Realised red-green colour blindness may be a problem here so added labels to lines Jan 2022.
