PURPOSE: Show how a quarter-wavelength coating prevents reflection.

DESCRIPTION: This is a dielectric coating with an index of refraction between that of air and glass, that covers one-half of the glass plate. If the coating is one-quarter wavelength thick for yellow light it prevents reflection of yellow light because the reflections from the two surfaces are exactly out of phase.

Light from a bright point source with a condenser lens and iris is focused by a 20 cm focal length convex lens through a glass plate onto a distant screen. In the photo above the glass plate reflects some light which is reflected a second time by a front surface mirror to form a spot next to the direct beam from the bright point source.

With no anti-reflective coating the direct beam is less intense because of the reflected beam. When the anti-reflective coating is raised into the beam the direct beam is more intense and the reflected beam is much less intense. When the yellow filter is used the beam reflected by the anti-reflective coating is slightly magenta colored, because the thickness of the coating is not quite one-quarter wavelength for the extreme colors of the spectrum.



EQUIPMENT: Bright point source with condenser lens and iris, 20cm focal length convex lens, glass plate with bottom half of surface toward source coated with one-quarter wavelength of dielectric coating, front surface mirror.

SETUP TIME: 5 min.

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