Model check. This lesson uses exact small training values (not measured constants) so every relationship is checkable to the digit. Real instruments report measured values with units, significant figures, and uncertainty; the idealized numbers here isolate the rule, not a measurement.
The same film path can accept or reject depending on the phase-flip target. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Dark with one phase flip closes at the whole-order target
The film path stays four metres. The mode is dark and the phase-flip bit is 1, making the target 4 m. The row is accepted.
P=2⋅2⋅1m=4mT=(2+0)2m=4mA=1
Dark without the flip shifts to a half-order target
The film path stays four metres. The mode is dark and the phase-flip bit is 0, making the target 5 m. The row is rejected.
P=2⋅2⋅1m=4mT=(2+21)2m=5mA=0
Bright without the flip returns to the whole-order target
The film path stays four metres. The mode is bright and the phase-flip bit is 0, making the target 4 m. The row is accepted.