Question

Opera glasses have a minimum length of 20 cm and a magnifying power of 5 when viewing distant objects. The focal lengths of lenses used are: 

Solution

Correct option is

25 cm, – 5 cm

 

Opera glasses are nothing but a Galelian telescope. For a Galelian telescope,

                          

  

Solving, we get:     fe = 5 cm, f0 = 25 cm

In a Galelian telescope, the eye-piece is a concave lens and hence f0 = 25 cm and fe = –5 cm.  

SIMILAR QUESTIONS

Q1

When the length of a microscope increases its magnifying power:

Q2

The focal length of the objective and the eyepiece of a compound microscope are 1 cm and 5 cm respectively. An object is placed at a distance of 1.1 cm from the objective. If the final image is formed at the least distance of distinct vision, the magnifying power is:        

Q3

In a compound microscope the objective and eye-piece have focal lengths of 0.95 cm and 5 cm respectively, and are kept at a distance of 20 cm. The last image is formed at a distance of 25 cm from eye-piece. What is the total magnification of the microscope?       

Q4

A person cannot see objects clearly beyond 50 cm. The power of the lens to correct the vision is:

Q5

A presbyopic patient has near point as 30 cm and far point as 40 cm. The dioptric power for the corrective lens for seeing distant objects is:     

Q6

A terrestrial telescope is made by introducing an erecting lens of focal length f between the objective and eye-piece lenses of an astronomical telescope. This causes the length of the telescope tube to increase by an amount equal to:       

Q7

The length of a telescope is 36 cm. The focal lengths of its lenses can be

Q8

An astronomical telescope of ten-fold angular magnification has a length of 44 cm. The focal length of the objective is: 

Q9

A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focused on a distant object in such a way that parallel rays emerge from the eye lens. If the object subtends an angle of 2o at the objective, the angular width of the image is:       

Q10

An astronomical telescope having an objective of focal length 100 cm is focused on the moon. Find the distance through which the eye-piece should be pulled back to focus an object situated at a distance 80 m from the objective: