Higher diffraction order eventually asks for a sine marker beyond the endpoint. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
First order fits inside the sine scale
Aperture width stays five metres and wavelength stays two metres. Order 1 needs target 2 m. The shown sine marker gives 2 m, so the row is accepted.
5m⋅52=2mmλ=1⋅2m=2mA=1
Second order still fits exactly
Aperture width stays five metres and wavelength stays two metres. Order 2 needs target 4 m. The shown sine marker gives 4 m, so the row is accepted.
5m⋅54=4mmλ=2⋅2m=4mA=1
Third order would need more than endpoint sine
Aperture width stays five metres and wavelength stays two metres. Order 3 needs target 6 m. The shown sine marker gives 5 m, so the row is rejected.