Experimental Determination of Focal Length of Lens

The focal length in $\mathrm{cm}$ of a biconvex lens of radii of each surface $50\mathrm{cm}$ and refractive index $1.5$ is

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The focal length of a biconvex lens of radii of each surface $50\mathrm{cm}$ and refractive index $1.5$ is

Air bubble in water behaves as,

A converging on a screen when the upper half of the lens of covered by opaque screen.

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In an experiment a convex lens of focal length 15cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20cm from the lens. The focal length of the convex mirror is: 

Graph of position of image vs position of a point object from a convex lens is shown in the figure. The focal length of the lens is-