Question

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:       

Solution

Correct option is

 

When the telescope is focused to moon, the objective forms the image at a distance of 100 cm from it. When the telescope is focused at an object at 

            

   

As the image is shifted (100/79) cm towards the eye-piece, it should be pulled back through (100/79) cm to focus the image in normal adjustment.

SIMILAR QUESTIONS

Q1

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:        

Q2

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?       

Q3

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

Q4

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:     

Q5

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:       

Q6

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

Q7

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

Q8

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:       

Q9

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: 

Q10

In a terrestrial telescope the focal length of erecting lens is 2 cm. The length of the telescope is 96 cm. If the magnifying power of the telescope is 10, then the focal lengths of eye-piece and objective are respectively: