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Diffraction
- wave phenomenon of light
- ability of light to bend around edges of an obstruction or through a narrow opening
- all waves are subject to diffraction when they encounter obstruction/aperature/irregularity in medium
- creates a secondary wave front
- out of phase with the primary wave
- interference with the primary wave front
- changes direction of the wave - corresponds to bending of light ray
- shorter the wavelength (e.g. blue light) = reduced diffraction
- smaller aperture = increased diffraction
Airy Disc
- when light passes through a circular aperture - circular diffraction pattern produced.
- central disc surrounded by alternating dark and light rings
- image formed on the retina from a distant source takes form of concentric light and dark rings surrounding a bright central disc
- Diameter (d) = 2.44f (focal length) / diameter of pupil
- longer wavelength - large diameter of Airy disc
- optical system free of aberration will form an airy disc on image plane.
Limits of resolution - resolving power
- smallest angle of separation between 2 points
- allows formation of 2 discernible images by the optical system.
- reached when 2 Airy discs separated such that the centre of 1 falls on the dark ring of the other.
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