Finding Focal Length Of A Convex Lens

The image obtained while finding the focal length of a convex lens is ……………… .

How would you find the focal length of a convex lens rapidly but approximately?

Find the focal length of a convex lens which produces a virtual image at 10 cm from the lens when an object is placed at 5 cm from the lens.

Find the focal length of a convex lens which produces a real image at 60 cm from the lens when an object is placed at 40 cm in front of the lens.

Define first principal focal length of a convex lens.

What is the combined focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm ? What is the nature of the combined lens ?

The focal length of a convex lens is