[" For the system given "],[" below,if the image of "],[" the object is formed "],[" on itself.What will be "],[" the focal length of the "],[" lens? "],[qquad ubrace(o)_(20cm)]

A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. The focal length of the lens is:

A 50 cm tall object is at a very large distance from a diverging lens. A virtual, erect and diminished image of the object is formed at a distance of 20 cm in front of the lens. How much is the focal length of the lens?

A convex lens forms a real image on a screen placed at a distance 60 cm from the object. When the lens is shifted towards the screen by 20 cm , another image of the object is formed on the screen. The focal length of the lens is :

A cocave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification produced is 1/4. the focal length of the lens will be

An object is kept at 60 cm in front of a convex lens. Its real image is formed at 20 cm from the lens . Find the focal length and power of the lens.

A lens forms a real image 3 cm high of an object 1 cm high. If the separation of object and image is 15 cm, find the focal length of the lens.

An object is kept at a distance of 16 cm from a thin lens and the image formed is real. If the object is kept at a distance of 6 cm from the lens, the image formed is virtual. If the sizes of the images formed are equal, the focal length of the lens will be

A concave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification produced is 1//4 . The focal length of the lens will be

The same size images are formed by a convex lens when the object is placed at 20cm or at 10cm from the lens. The focal length of convex lens is ____ cm