Displacement Method
Category : JEE Main & Advanced
By this method focal length of convex lens is determined.
Consider an object and a screen placed at a distance \[D(>4f)\] apart. Let a lens of focal length f be placed between the object and the screen.
(1) For two different positions of lens two images \[({{I}_{1}}\text{ and }{{I}_{2}})\]of an object are formed at the screen.
(2) Focal length of the lens \[f=\frac{{{D}^{2}}-{{x}^{2}}}{4D}=\frac{x}{{{m}_{1}}-{{m}_{2}}}\]
where \[{{m}_{1}}=\frac{{{I}_{1}}}{O}\,\]; \[{{m}_{2}}=\frac{{{I}_{2}}}{O}\] and \[{{m}_{1}}{{m}_{2}}=1.\]
(3) Size of object \[O=\sqrt{{{I}_{1}}.\,{{I}_{2}}}\]
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