For green light, the diameter is 0.00124 mm times the focal ratio. The first is the linear size as if we had an incredibly precise ruler. What is the size of the Airy disk? There are two ways to measure the Airy disk. Make the slit a circular opening and now the fuzzy image of the slit becomes a fuzzy disk surrounded by ever fainter rings. Projected on the wall will be a fuzzy image of the slit (the equivalent of the Airy disk), surrounded with ever fainter straight bands. Think of the famous slit experiment where light or electrons, anything that has wave properties travel through a slit. Why a discrete spot size and rings? Light diffracts because it has wave properties. Light going through a circular opening forms a spot of light surrounded by ever fainter rings. What is the Airy disk? The smallest spot of light that a perfect lens or mirror can focus is called the Airy disk.
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