Lecture 5 - Lecture 5 Lecture 5 Thin Lens Model We now...

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Lecture 5 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 5 Thin Lens Model We now construct our model for the thin lens in air. Lens index is . We will find the imaging proper- ties of the thin lens by using the previous results for a single spherical surface and applying them twice - once for each of the two surfaces of the lens. Use Eq. (4.12) for first surface: , , (5.1) We get a virtual object at Now consider the rays travelling inside the lens from the virtual object. Apply spherical surface law now to the second surface. This time , , , (5.2) The thin lens approximation is that the lens thickness is negligible, so that . Using this in Eq (5.1), then substituting in Eq. (5.2), (5.3) This is the Gaussian lens law, with the focal length identified as: (5.4) This is called the lensmaker’s equation . n l d 1 R 2 R 1 n l d 1 d 2 d 2 n 1 = n n l = ld 1 l d 1 n l d 1 ------- 1 d 1 ----- n l 1 R 1 ------------- = d 1 n 1 = nn l = 2 l d 2 1 d 2 n l d 2 ------ 1 n l R 2 = d 2 d 1 1 d 2 1 d 1 1 n l R 2 n l 1 R 1 + = 1 f -- n l 1  1 R 1 1 R 2   =
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Lecture 5 2 © Jeffrey Bokor, 2000, all rights reserved We conclude that a lens with 2 spherical surfaces satisfies the Gaussian lens law, but only under 2 important approximations Paraxial approximation Thin-lens approximation
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Lecture 5 - Lecture 5 Lecture 5 Thin Lens Model We now...

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