An achromatic telescope objective is to be made by combining the lenses of flint and crown glasses. This proper choice is

The radius of curvature of each surface of a convex lens having refractive index $1.8$ is $20 \ cm$. The lens is now immersed in a liquid of refractive index $1.5$. The ratio of power of the lens in air to its power in the liquid will be $x : 1$. The value of $x$ is $.....$

The relation between $n_1$ and $n_2$,if the behavior of light rays is as shown in the figure,is:

Assertion : Position of image approaches focus of a lens,only when object approaches infinity.
Reason : Paraxial rays incident parallel to principal axis intersect at the focus after refraction from lens.

$A$ thin lens with focal length $f$ is to be used as a magnifying glass. Which of the following statements regarding the situation is true?

Consider the following statements regarding the real images formed with a converging lens.
$I$. Real images can be seen only if the image is projected onto the screen.
$II$. The real image can be seen only from the same side of the lens as that on which the object is positioned.
$III$. Real images produced by converging lenses are not only laterally but also longitudinally inverted as with mirrors.
Which of the above statement$(s)$ is/are incorrect?