Q8 of 20 Page 304

A person is unable to see objects nearer than 50 cm. He wants to read a book placed at a distance of 25 cm. Find the nature, focal length and power of the lens, he requires for his spectacles.

Object distance, u = 25 cm

The distance of near point, d = 50 cm


Focal length, f = ??


Using the formula: f =


f = = = 50 cm


Power = = = 2D


As focal length is positive therefore he uses double convex lens.


More from this chapter

All 20 →
6

Four students A, B, C and D were asked by their teacher to find the corresponding image distance for the given object distance keeping the object at various distance in case of a given convex lens of focal length 15 cm. The following observation were recorded by them.


Which student recorded the incorrect observation?

 7

A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect Draw ray diagram for the:

i. defect of vision and also


ii. for its correction.

 9

A person with a myopic eye cannot see objects beyond a distance of 1.5 m from it. What would be the nature and power of the corrective lens used to restore proper vision?

 10

a. What is the least distance of distinct vision for a normal eye?

b. Does the above distance increase or decrease for a long- sighted eye? Give reason for your answer with diagram.