If a real and inverted image is formed by a convex lens on a screen 20 cm away from the lens, and the object is placed 30 cm from the lens, what is the focal length of the lens according to the lens formula?

If a lens has a focal length of -0.2 m, what is its optical power and type of lens?

The magnifying power of an astronomical telescope is 10 and the focal length of its eyepiece is 20 cm. The focal length of its objective lens will be:

A convex mirror has a radius of curvature of 60 cm. If an object is placed 40 cm in front of the mirror, find the distance of the image from the mirror.