A Polaroid only admits light whose direction of polarization is along its transmission axis.
Work Step by Step
When the two Polaroids are aligned, anything that passes through the first gets through the second with no problem. When the two Polaroids are at right angles to each other, anything that passes through the first is automatically absorbed by the second. This is discussed on pages 555-556, and an analogy with ropes passing through a pair of picket fences is shown in Figure 29.32.