An object is kept at a distance of 18 cm, 20 cm, 22  cm and 30 cm, from a lens of power + 5D.  


(a) The focal length of the given lens is: 

(1) 0.2 cm 

(2) 20 cm  

(3) 5cm 

(4) 0.5 cm  



Focal length of a lens = 1/Power 

 = ⅕ = 0.2 m 

Note: focal length obtained from the power formula is always in metres .


Thus, the focal length of given lens is 20cm .

So, the correct answer is (ii) 

(b) In which case or cases would you get a magnified  image? 

(i) 18cm 

(ii) 20cm  

(iii) 22cm 

(iv) All the above  




  • Focal length is 20 cm
  • And focal length is positive
  • It is a convex lens

Now let us go through the options. 

  • (i) 18cm - Object between F and P: The image produced is virtual, erect and magnified.
  • (ii) 20cm -  Object at F: The image produced at F is real, inverted  and magnified. 
  • (iii) 22cm - Object between C and F: The image will be real, inverted and magnified. 

Since magnified image is being formed at all object distances.


Thus, the correct answer is (iv) all the above

(c) Which of the magnified images can we get on a screen? 

(i) At 18 cm  

(ii) At 18 cm and 22 cm  

(iii) At 20 cm and 22 cm  

(iv) At 22 and 30 cm  



As explained in question (b), for convex lens

  • The object at 18 cm forms a virtual image . This cannot be captured on a screen.

  • The object at 20 cm forms an image at infinity . Since the rays never meet, the image cannot be caught on a screen.

  • Only the object at 22 and 30 cm form real images with rays that converge to form an image. Hence, these images can be captured on a screen. 


Thus the correct answer is (iv) at 22 cm and 30 cm


(d) The power of a lens of focal length 1 metre is  called as:  

(i) Refractive index 

(ii) 1 dioptre  

(iii) Speed of light 

(iv) Wavelength  




(e) Name the lens/mirror used in film projectors and  telescopes:  

(i) Convex lens 

(ii) Concave lens  

(iii) Convex mirror 

(iv) Concave mirror  


Ans. Let us go through our options:

  • A Concave lens or a convex mirror produce virtual images. These cannot be used in film projectors as images cannot be captured on a screen. 
  • A convex lens and concave mirror produce real, magnified images (everywhere except between F and P ) that can be captured on screen. 


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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo