What is the power of a concave lens of focal length $25\, cm$?

What is the absolute refractive index of water?

What is a prism?

Which mirror is used by dentists?

In an astronomical telescope,how is the focal length of the objective lens compared to the focal length of the eyepiece?

Identify the device used as a spherical mirror or lens in the following cases,where the image formed is virtual and erect in each case.
$(a)$ Object is placed between the device and its focus,image formed is enlarged and behind it.
$(b)$ Object is placed between the focus and the device,image formed is enlarged and on the same side as that of the object.
$(c)$ Object is placed between infinity and the device,image formed is diminished and between the focus and optical centre on the same side as that of the object.
$(d)$ Object is placed between infinity and the device,image formed is diminished and between the pole and focus,behind it.