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?

A double convex lens of focal length 'F' is cut into two equal parts along the vertical axis (passing through the optical center). The focal length of each individual part will be:

According to Fermat's principle, when a light ray refracts while traveling from one medium to another medium of a different refractive index, the actual path chosen is the path of: