Lloyd's mirror
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Lloyd's mirror is a classic optics experiment and was first described in 1834. Place a converging lens about 10cm in front of a diverging lens. Shine a laser into both. Place a screen about three meters from the laser and adjust the position of the two lenses so that the smallest possible spot can be seen on the screen. Lay a microscope slide between the two lenses. Carefully raise the slide until the laser beam just grazes its upper surface. A second spot will appear on the screen, about one inch above the first. Remove the converging lens without disturbing the other apparatus. With this lens removed, the cones of light coming from the direct and reflected sources partially overlap, forming an interference pattern on the screen.
This interference pattern was also used with radio telescopes located along the Australian coastline, using the sea instead of the microscope slide in the optical experiment. This allowed the first interferometric measurements of astronomical objects.
