Displacement of a beam passing through a thick plate of glass.

Displacement of a beam passing through a thick plate of glass. On a small turntable, a vertical metal index pin is placed in front of a 5x5x5 cm glass cube. Collimated white light is sent past the pin and through the glass cube and is then focused on a projection screen with a lens. (The Dove erecting prism is used to make an upright image.) Upon rotating the cube, the projected image of the pin through the cube is offset from the image of the pin. See figure 1. Figure 1A shows the light normal to the cube. There is no displacement of the image of the pin seen through the cube. In figure 1B, the cube is rotated, and the image of the pin seen through the cube is displaced with respect to the image of the pin alone. See figure 2. Light traveling through air is incident at an angle i on side 1 of a plate of plane-parallel glass. It is refracted into the glass at angle r. The light is incident on side 2 at angle r, and is refracted into the air at angle i. Thus, the exiting beam is parallel to the incident beam, but displaced. Ref: Modern College Physics by Harvey White, 6th ed., p.490-491