As shown in figure, a convergent lens is placed inside a cell filled with liquid. The lens has focal length +20cm when in air, and its material has refractive index 1.50. If the liquid has refractive index 1.60, the focal length of the system is: 



Correct option is

– 100 cm






Focal length of a convex lens will be maximum for: 


A concave mirror is focal length f in air is used in a medium of refractive index 2. What will be the focal length of the mirror in the medium? 


A luminous object is placed at a distance of 30 cm from a convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens must a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it


The far-point of a short-sighted eye is 200 cm. The power of the lens is:


The near point of a short-sighted person is 10 cm and he desires to read a book 30 cm away from him. The power of the lens to be used by him is:


The distance between an object and the screen is 100 cm. A lens produces an image on the screen when placed at either of the positions 40 cm apart. The power of the lens is: 


A thin equiconvex lens has focal length 10 cm and refractive index 1.5. One of its faces is now silvered and for an object placed at a distance u in front of the lens, the image coincides with the object. The value of u is:


The plane face of a plano-convex lens of focal length 20 cm is silvered. What type of mirror will it become and of what focal length 


A convex lens of focal length 20 cm is cut into two equal parts so as to obtain two plano-convex lenses as shown in figure. The two parts are then put in contact as shown in figure. What is the focal length of the combination?



A plano-convex lens of f = 20 cm is silvered at plane surface. Now, f will be (μ = 1.5):