# JEE Main & Advanced Physics Ray Optics Displacement Method

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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