What is the focal length of the objective of an astronomical telescope as compared to that of the eyepiece?

The image formed by a concave mirror is observed to be virtual,erect and larger than the object. Then the position of the object should be

What type of image is always formed by a concave lens?

Which of the lenses with focal lengths $10 \, cm$,$20 \, cm$,$25 \, cm$,and $50 \, cm$ has the maximum power (in $, cm$)?

Draw ray diagrams showing the image formation by a convex lens when an object is placed:
$(a)$ Between optical centre and focus of the lens.
$(b)$ Between focus and twice the focal length of the lens.
$(c)$ At twice the focal length of the lens.
$(d)$ At infinity.
$(e)$ At the focus of the lens.

What are the advantages and disadvantages of using a convex mirror for seeing traffic at the rear?