Lecture16 - Lecture 16: More on Lenses Last time we began a...

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Last time we began a discussion of using refraction to form an image: As long as t is small, we showed that: 1 s + 1 σ = ν -1 ( 29 1 Ρ 1 - 1 2 Lecture 16: More on Lenses t R 1 R 2 s Object Image
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Focal length As with a spherical mirror, we can define the focal length of the lens as the point at which the image of an object infinitely far away would appear: With this definition, the lens-maker’s equation looks just like the mirror equation: 1 + 1 φ = ν -1 ( 29 1 Ρ 1 - 1 2 1 ( 29 1 1 - 1 2 1 s + 1 σ = 1 Note: a lens has two focal points (one on each side of the lens), but both have the same focal length
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Converging and Diverging Lenses We can separate lenses into two classes based on what happens to parallel rays of light that pass through them: 1. Converging lens (thickest at the middle): 1. Diverging lens (thickest at the edges):
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Sign Conventions As usual for geometric optics, we need to understand the signs of all the quantities Quantity + when… - when… s Object is real (in front of lens) Object is virtual (behind lens) Image is real (behind lens) Image is virtual (in front of lens) Image is upright Image is inverted R 1 and R 2 Center of curvature is behind lens Center of curvature is in front of lens f Lens is converging Lens is diverging y
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Magnification Conveniently, the magnification of an image by a lens is given by the same expression as that for a mirror: So when m is positive, the image is upright and on the same side of the lens as the object When m is negative, image is inverted and on opposite side of lens m = ψ = - σ
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Example An antelope is 20m away from a converging lens with f = 30cm.
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This note was uploaded on 08/24/2010 for the course PHYS 142 taught by Professor Staff during the Fall '08 term at University of Arizona- Tucson.

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Lecture16 - Lecture 16: More on Lenses Last time we began a...

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