Question

A person can see clearly only up to a distance of 30 cm. He wants to read a book placed at a distance of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?   

Solution

Correct option is

–1.33 D

 

The person is myopic. This defect is correct by using a concave lens. Putting u = –50 cm and v = –30 cm in the lens formula  fives f = –75 cm. Therefore, the power of the lens

                           = – 1.33 D.  

SIMILAR QUESTIONS

Q1

A prism of refractive index of  has a refracting angle of 60o. At what angle must a ray be incident on it so that it suffers a minimum deviation?

Q2

A ray of light in a medium of refractive index μ1 is partly reflected and refracted at the boundary of a medium of refractive index μ2, as shown in fig. If BOC = 90o. The value of angle i is given by 

                                                           

Q3

A rectangular block of glass of refractive index μ and thickness d is placed over an ink dot made on a pice of paper lying on a table. When viewed from above, the dot will appear to be raised by an amount given by

Q4

Two transparent media A and B are separated by a plane boundary. The speed of light in medium A is  and in medium B  The critical angle for which a ray of light going from A toB is totally internally reflected is   

Q5

A convex glass lens (μg = 1.5) has a focal length of 8 cm when placed in air. What is the focal length of the lens when it is immersed in water (μw = 4/3)?

Q6

A plano-concave lens is made of glass of refractive index 1.5 and the radius of curvature of the curved face is 50 cm. The power of the lens is

Q7

A convex lens of focal length 50 cm and a concave lens of focal length 100 cm are separated by a distance of 10 cm. What is the power of the combination?    

Q8

How will the image formed by a convex lens be affected if the central portion of the lens is wrapped in black paper, as shown in fig.

                                                                            

Q9

The equi-convex lens, shown in the figure, has a focal length f. What will be the focal length of each half if the lens it cut along AB? (See fig.)

                                                                   

Q10

A person with defective eyesight is unable to see objects clearly nearer than 1.5 m. He wants to read a book placed at a distance of 30 cm from his eyes. Find the power of the lens he requires for his spectacles.