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  

 

Answer

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  

 

Answer

Since

  • 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  

 

Ans. 

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  

 

Answer

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(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. 

 

  1. Class 10
  2. Chapter 10 Class 10 - Light - Reflection and Refraction (Term 1)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo