A convex surface, like my rainbow maker’s mirror, reflects light in a curved shape. For example, if my rainbow maker was parallel to a wall all the parts of the mirror would be the same distance from the wall, but the light reflecting from the lower parts of the mirror wouldn’t reach as high up the wall as the light from the higher mirror.
If I positioned the rainbow maker at the correct angle, one of the lower parts of the mirror would be able to reflect light to the same height up the wall as the highest section of the mirror.