10_ retarder plate makes dark bands in spectrum, using arc lamp.

10 _ retarder plate makes dark bands in spectrum using arc lamp. Collimated white polarized light is sent through a slit, a 10 _ retarder plate, another Polaroid, and then a diffraction grating (19,050 lines-per-inch), and the resulting spectrum is focused on a projection screen. There are four dark bands in the spectrum, which move as the second Polaroid is rotated. The retarder plate retards green light (5600 A) exactly 10_, but retards blue light (5100 A) about 11_, and red light (6200 A) about 9_. Initially, each Polaroid is placed with its axis at 45 deg WRT the retarder plate, and there is no _/4 plate. See figure 1. The spectrum is now composed of regions of plane and circularly polarized light, and figure 2A shows 4 dark bands where light is blocked by the second Polaroid. 2B shows how the placement of the dark bands if the second Polaroid is rotated 90 deg . 2C shows the case where the second Polaroid is vertical: 50% of all polarized light is transmitted. Figure 2D shows how, when a _/4 plate is inserted at point 6 with the slow or fast axis vertical, the bands are shifted a small amount left or right.