An object to the left of a lens is imaged by the lens on a screen `30cm` to the right of the lens. When the lens is moved ` 5cm` to the right, the screen must be moved `5cm` to the left to refocus the images. The focal length of lens is :-

A convex lens forms an image of an object on a screen 30 cm from the lens. When the lens is moved 90 cm towards the object, then the image is again formed on the screen. Then, the focal length of the lens is

When an object is placed at the proper distance ot the left of a converging lens, the image is focused in a screen 30 cm to the right of lens. A dieverging lens is now placed 15 cm to the right of the converging lens and it is found that the screen must be moved 19.2cm farther to the right to obtain a sharp image. Find the focal length of the diverging 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 convex lens produces an image of a real object on a screen with a magnification of 1/2. When the lens is moved 30 cm away from the object, the magnification of the image on the screen is 2. The focal length of the lens is

An object placed at 20 cm from a lens, forms an image on a screen placed 60 cm on the other side of the lens. What is the focal length and type of the lens ?

An object is 60 cm in front of a thin lens, the image being 300 cm on the other side of the lens. Calculate the displacement of the image, when the object is moves 20 cm nearer to the lens.

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:

Aluminous object and a screen are placed on an optical bench and a converging lens is placed between them to throw a sharp image of the object on the screen, the linear magnification of the image is found to be 2.5. The lens is now moved 30 cm nearer the screen and a sharp image is again formed. Calculate the focal length of the lens.

An object and a screen are mounted on an optical bench and a converging lens is placed between them so that a sharp image is obtained on the screen. The linear magnification of the image is 25. The lens is now moved 30 cm towards the screen and a sharp image is ■ again formed on the screen. Find the focal length of the lens

A needle of height 5 cm placed 45 cm from a lens forms an image on a screen placed 90 cm on the other side of the lens the type of lens and its focal length are