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?



Correct option is

20 cm


A single double convex lens may be regarded as a combination of two plano-convex lenses. Let be the focal length of each plano-convex part. Then   


In the problem, again two plano-convex lenses each of focal length 40 cm are put in contact. Now, the combined focal length is given by  




A real image is formed by a convex lens. If we put it in contact with a concave lens and the combination again forms a real image, which of the following is true for the new image from the combination?  


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 


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: