If all ray aberrations in an optical system can be eliminated, such that all The radius r of the Airy Disc at the focal point of a lens is given by The size of the Airy Disc is determined by the focal length f and diameter D of Light near the focal point exhibits an Airy Disc pattern. This yields a blurred spotĪt the focal point. Light passing through the lens therefore spread out. Lens itself acts like an aperture with diameter D for the incident light. Parallel rays to a single point one focal length away from the lens. In geometrical optics we assume that an ideal, aberration-free lens focuses The diameter increases by a factor of 10. Consider a HeNe laser, for which λ = 633 nm with a beam waist of ~ 0.6 mm. Typically much larger the wavelength of light, or a(0) > λ, θ is quite small.Īt a large distance z the diameter of the beam will have increased to a(z) ≈ z*2θ. Often ignore the factor of 1.22.) Because the laser beam diameter is (For back-of-the-envelope calculations we The angle through which the light spreads is approximately Same way it does after passing through an aperture.Īssume that at z = 0 the diameter of a laser beam is restricted to a(0). Transverse to the direction of propagation. Producing a laser beam is an attempt to confine the light in the directions The minimum is seen at a radial distance r' = 1.22 λL/D from the center of the pattern On a screen a distance L > D from the aperture Theįirst minimum occurs at an angle θ = 1.22 λ/D, where D is The main features are shown in the diagram below. The intensity pattern is called the "Airy Disk". Diffraction of light through a rectangular aperture isĪ rather straightforward extension of 1-dimensional diffraction from aĪ circular aperture is qualitatively similar, but anĪccurate quantitative treatment of the pattern requires more complicated
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