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| Lens Formula |
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| This gives the relation between focal length, object distance and image distance from the optical centre of the lens. |
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| Consider an object AB held perpendicular to the principal axis at distance beyond the focal length of the lens. A real, inverted and enlarged image is formed as shown. |
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| But CD=AB |
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| It follows |
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| BC = -u |
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| BlC = +v using sign convention |
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| CF = +f |
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| FBl = CBl - CF = v - f |
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| vf = -uv + uf |
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| uv = uf - vf |
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| Dividing throughout by uvf |
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| Note: (1) The above formula is applicable for a convex lens when a virtual image is formed. For this the following ray diagram is to be considered. |
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| 2) For concave lens, the ray diagram would be |
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| Real Image |
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| Virtual Image |
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