A mirror reverses the photon's momentum rather than stopping it, so the momentum handed to the surface doubles, giving P = 2E / c. The factor of two is the counting difference between absorption and reflection. It is exact only for a perfect reflector at normal incidence with stated irradiance; real mirrors absorb and scatter, and beam power and alignment carry measurement error.
A perfect mirror uses the same source but a doubled pressure multiplier. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A perfect mirror doubles the pressure
The mirror case uses the same irradiance and speed with multiplier two.
pmirror=2cI=2Pa
The absorber value stays one pascal
The doubled value is not copied from the absorber label.