Question

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 

Solution

Correct option is

Concave, f = 10 cm

When the object is placed at the focus of the lens, the refracted rays will be incident normally on the silvered surface. So, they will retrace their path.

                                                                      

Hence, the image will be formed at the location of the object. In this way, the combination behaves as a concave mirror of radius of curvature (R) = 20 cm.   

                                                                                     

In absence of concave lens, the parallel beam will be focused at F2, i.e., at a distance of 20 cm from the lens A. The focal length of the concave lens is 5 cm, i.e., if this lens is placed at 5 cm from F2, then the beam will become parallel. So, d = 15 cm.   

SIMILAR QUESTIONS

Q1

Given  There is an equiconvex lens with radius of each surface equal to 20 cm. There is air in the object space and water in the image space. The focal length of lens is:

Q2

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?  

Q3

Focal length of a convex lens will be maximum for: 

Q4

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? 

Q5

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

Q6

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

Q7

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:

Q8

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: 

Q9

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:

Q10

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?