The key to the optical rotary encoder is that, as the wheel is rotated, the light emitted from the IR LED is intermittently blocked by the encoding disk.
However, it is possible to have an absolute optical encoder which has a disk that looks like this, where different angles have different sections open to let the light through.
First, the incremental optical encoder from earlier can't read out an absolute angular position because the wheel has 48 equally spaced spokes, and it can't discern one angle from the next.