The same size images are formed by a convex lens when the object is placed at $20\, cm$ or at $10\, cm$ from the lens. The focal length of the convex lens is ............ $cm$.

Focal length of a convex lens will be maximum for

The focal length of a convex lens is $f$. An object is placed at a distance $x$ from its first focal point. The ratio of the size of the real image to that of the object is

If an object is placed at $A$ $(OA > f)$,where $f$ is the focal length of the lens,the image is found to be formed at $B$. $A$ perpendicular is erected at $O$ and $C$ is chosen on it such that the angle $\angle BCA$ is a right angle. Then the value of $f$ will be

$A$ convex lens has its radii of curvature equal. The focal length of the lens is $f$. If it is divided vertically into two identical plano-convex lenses by cutting it, then the focal length of the plano-convex lens is ($\mu =$ the refractive index of the material of the lens).

Assertion: Goggles have zero power.
Reason: Radius of curvature of both sides of the lens is same.