A convex lens forms a real image $9cm$ long on a screen. Without altering the position of the object and the screen, the lens is displaced and we get again a real image $4cm$ long on the screen. Then the length of the object is-

Q. A convex lens makes a real image $4cm$ long on a screen. When the lens is shifted to a new position without disturbing object or the screen, we again get a real image on the screen which is $9cm$ long. The length of the object must be :

Q. A convex lens when placed in the first position forms a real image of an object on a fixed screen. The distance between the object and the screen is $75\text{cm}$. On displacing the lens from first position by $25\text{cm}$ to the second position, again a real image is formed on the screen. Then the focal length of the lens is

Q. A convex lens is used to form real image of an object on a screen. It is observed that even when the positions of the object and that screen are fixed are two position of the lens to form real images. If the heights of the images are $4cm$ and $9cm$ respectively the height of the object is

Q. A convex lens forms a real image three times larger than the object on a screen. Object and screen are moved until the image becomes twice the size of the object. If the increase in distance of the object is 6 cm, the shift of the screen is: