Base on the french text, I assume the 10 degree is the full cone angle since it says "the reflected beam have a 10 degree angle", but I may misinterpreted it
Yes... I think there is some line like the 21.8 and 68 degree angle, and the 2 really light gray line. The original question is in french, and it's for my daughter. Some of my Physics and Geometry are quite rusted.
Thank everyone, yes it's a spherical mirror, I don't think it's about aberration neither but could be wrong. I just don't see the relation of the virtual image position vs angle/diameter.
I've look if there was any way to get the "image size" or a ratio to use the Mirror Equation to find the focal length, but nothing. I think it's base on some geometry, but I don't see the relation.