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Thin-Lens Equation:Cartesian Convention

If the Cartesian sign convention is used, the Gaussian form of the lens equation becomes

because in that convention the positive direction for a quantity is always in the direction that light is traveling. The object distance o is then a negative number because to travel from the lens to the object, you must travel in the direction opposite to light travel. The thin lens equation is also sometimes expressed in the Newtonian form. The derivation of the Gaussian form proceeds from triangle geometry. For a thin lens, the lens power P is the sum of the surface powers. For thicker lenses, Gullstrand's equation can be used to get the equivalent power.

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