In the condition shown in the large diagram (Figure 1) part of the laser beam falls on a plateau and part on a pit. The pit is one quarter of a wavelength deep and so destructive interference results giving a zero. The following set of small diagrams (Figure 2) shows the change from zero to one in the reflected intensity.

Laser beam reflected from a plateau – constructive interference results and so the output signal is high – a one.

Laser beam reflected from the edge of a plateau – destructive interference results and so the output signal is low – a zero.

Laser beam reflected from a pit – constructive interference results and so the output signal is high – a one.

Laser beam reflected from the edge of a plateau – destructive interference results and so the output signal is low – a zero.

Laser beam reflected from a plateau – constructive interference results and so the output signal is high – a one.