wk2b - Lenses and Imaging (Part I) Why is imaging...

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MIT 2.71/2.710 09/15/04 wk2-b-1 Lenses and Imaging (Part I) • Why is imaging necessary: Huygen’s principle – Spherical & parallel ray bundles, points at infinity • Refraction at spherical surfaces (paraxial approximation) • Optical power and imaging condition • Matrix formulation of geometrical optics • The thin lens • Surfaces of positive/negative power • Real and virtual images
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MIT 2.71/2.710 09/15/04 wk2-b-2 Parabloid mirror: perfect focusing s ( x ) x z (e.g. satellite dish) () f x x s 4 2 = f f : focal length
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MIT 2.71/2.710 09/15/04 wk2-b-3 Lens: main instrument for image formation Point source ( object ) Point image The curved surface makes the rays bend proportionally to their distance from the “optical axis”, according to Snell’s law. Therefore, the divergent wavefront becomes convergent at the right-hand (output) side. air glass air optical axis
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MIT 2.71/2.710 09/15/04 wk2-b-4 Why are focusing instruments necessary? • Ray bundles: spherical waves and plane waves • Point sources and point images • Huygens principle and why we can see around us • The role of classical imaging systems
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MIT 2.71/2.710 09/15/04 wk2-b-5 Ray bundles point source spherical wave (diverging) point source very-very far away plane wave wave-front rays wave-front rays
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MIT 2.71/2.710 09/15/04 wk2-b-6 Huygens principle optical wavefronts Each point on the wavefront acts as a secondary light source emitting a spherical wave The wavefront after a short propagation distance is the result of superimposing all these spherical wavelets
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MIT 2.71/2.710 09/15/04 wk2-b-7 Why are focusing instruments necessary? object object ... is scattered by the object incident light
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MIT 2.71/2.710 09/15/04 wk2-b-8 Why are focusing instruments necessary? object object ... is scattered by the object incident light image image need optics to “undo” the effects of scattering, i.e. focus the light
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MIT 2.71/2.710 09/15/04 wk2-b-9 Ideal lens Point sources ( object ) Point images Each point source from the object plane object plane focuses onto a point image at the image plane image plane; NOTE NOTE the image inversion the image inversion air glass air optical axis
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MIT 2.71/2.710 09/15/04 wk2-b-10 Summary: Why are imaging systems needed? Each point in an object scatters the incident illumination into a spherical wave, according to the Huygens principle. A few microns away from the object surface, the rays emanating from all object points become entangled, delocalizing object details. To relocalize object details, a method must be found to reassign (“focus”) all the rays that emanated from a single point object into another point in space (the “image.”) The latter function is the topic of the discipline of Optical Imaging. ?
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MIT 2.71/2.710 09/15/04 wk2-b-11 The ideal optical imaging system optical elements Ideal imaging system: each point in the object object is mapped onto a single point in the image image object object image image Real imaging systems introduce blur . ..
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