What is the absolute refractive index of common glass?

$A$ beam of light is incident through the holes on side $A$ and emerges out of the holes on the other face of the box as shown in the Figure. Which of the following could be inside the box?

You are given a convex lens of focal length $10\, cm$. Where would you place an object to get a real,inverted and highly enlarged image of the object? Draw a ray diagram.

$(a)$ Define power of a lens and write its $SI$ unit.
$(b)$ $A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from it. Where is the needle placed in front of the lens, if the image size is equal to the object size? Also, find the power of the lens.

How will you decide whether a given piece of glass is a convex lens,concave lens or a plane glass plate?

The image formed by a lens is always erect and diminished. What is the nature of the lens?