An object is held at the principal focus of a concave lens of focal length F. where is the image formed?

Define the principal focus of concave mirror.

An object is at a distance of 5 m from a convex mirror of focal length 10 cm.Where is he image formed ?

Find the power of a concave lens of focal length 2 meters?

An object of size 3.0cm is placed 14cm in front of a concave lens of focal length 21cm. Describe the image produced by the lens. What happens if the object is moved further away from the lens?

An object is kept in front of a concave mirror of focal length 15 cm.The image formed is three times the size of the object.Calculate two possible distances of the object from the mirror.