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What is Airy's discs?...

What is Airy's discs?

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(i) Similar to a rectangular slit, when a circular aperture or opening (like a lens or the iris of our eye) forms an image of a point object, the image formed will not be a point but a diffraction pattern of concentric circles that becomes fainter while moving away from the center as shown in Figure. These are known as Airy's discs.
`a sin theta = 1.22 lamda`

(ii) Here, the numerical value 1.22 comes for central maximum formed by circular apertures.
For small angles, `sin theta = theta`
`a theta = 1.22 lamda`
Rewriting further,
`theta = (1.22 lamda)/(a) " and " (r_0)/(f) = (1.22 lamda) / (a)`
` r_0 = (1.22 lamda f)/( a) `
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