Which one of the following materials cannot be used to make a lens?

Why is a convex lens called a convergent lens?

The word KVPY is written on $\mathrm{a}$ board and viewed through different lenses such that board is at a distance beyond the focal length of the lens.

Ignoring magnification effects, consider the following statements.
$\left(\mathrm{I}\right)$ First image has been viewed from the planar side of a plano-concave lens and second image from the planar side of a plano-convex lens.
$\left(\mathrm{II}\right)$ First image has been viewed from the concave side of a plano-concave lens and second image from the convex side of a plano-convex lens.
$\left(\mathrm{III}\right)$ First image has been viewed from the concave side of a plano-concave lens and second image from the planar side of a plano-convex lens.
$\left(\mathrm{IV}\right)$ First image has been viewed from the planar side of a plano-concave lens and second image from the convex side of a plano-convexlens.

Which of the above statements are correct?

The following diagram shows the use of an optical device to perform an experiment of light.

As per the arrangement shown, the optical device is likely to be a _____.

What are the types of spherical lenses? Describe the principal focus, aperture and the optical centre of a spherical lens.

What is meant by the optical centre of a convex lens?