$A$ convex lens of focal length $f$ $m$ forms a real,inverted image twice the size of the object. The object distance from the lens in metres is:

$A$ convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens material. Then its focal length will

$A$ thin glass (refractive index $1.5$) lens has an optical power of $-5 D$ in air. Its optical power in a liquid medium with a refractive index of $1.6$ will be:

$A$ convex lens is used to form an image of an object on a screen. If the upper half of the lens is blackened,so that it becomes opaque,then

An object is placed first at infinity and then at $20 \ cm$ from the object-side focal plane of a convex lens. The two images thus formed are $5 \ cm$ apart. The focal length of the lens is $...... \ cm$.

Images of same size are formed by a convex lens when an object is placed either at $20 \ cm$ or $10 \ cm$ distance from the lens. The focal length of the lens is (in $cm$)