A student focussed the image of a candle flame  on a white screen by placing the flame at various  distances from a convex lens. He noted his  observations as:  

S. No.

Distance of flame  from the lens (cm)

Distance of the screen  from the lens (cm)

(i)

60

20

(ii)

40

24

(iii)

30

30

(iv)

24

40

(v)

15

70





(a) From the above table, find the focal length of lens  without using lens formula:   

(i) 15cm 

(ii) 30cm  

(iii) 40cm 

(iv) 60cm  

 

Answer

We know that in a Convex lens

  • If an object is kept at 2F , it will form at 2F

 

From table, we can see that

  • Distance of image and Distance of object i s the same at 30 cm
  • Thus, 2F = 30 cm
  • So, Focal length = F = 15 cm

 

So, the correct answer is (i) 15 cm



(b) Which set of observations is incorrect and why?   

(i) Object distance can not be smaller than image distance  

(ii) Image should form at 15 cm only  

(iii) Image should form at infinity  

(iv) Object is kept at optical centre  

 

Answer

S. No.

Distance of flame  from the lens (cm)

Distance of the screen  from the lens (cm)

(i)

60

20

(ii)

40

24

(iii)

30

30

(iv)

24

40

(v)

15

70

We check all observations one-by-one, for a convex lens of focal length 15 cm

  • For observation (i) - Object is kept beyond 2F , so distance of image will be between F and 2F, i.e. between 15 and 30 cm . Since, it is 20 cm, this observation is correct

  • For observation (ii) - Object is kept beyond 2F , so distance of image will be between F and 2F, i.e. between 15 and 30 cm . Since, it is 24 cm, this observation is correct

  • For observation (iii) - Object is kept at 2F , so distance of image will also be 2F i.e. image distance is 30 cm . Since, it is 30 cm, this observation is correct

  • For observation (iv) - Object is kept between F and 2F , so distance of image will be beyond 2F i.e. image distance is beyond 30 cm . Since, it is 40 cm, this observation is correct

  • For observation (v) - Object is kept at F (i.e. 15 cm) , so distance of image will be at infinity i.e. image distance cannot be calculated . Since, it is 70 cm, this observation is incorrect

 

Now, in our options

  • In Observation (v), image should be formed at infinity and not at 70 cm. So, this is not correct



So, the correct option is (iii) - Image should form at infinity  




(c) In which case, the size of the object and image  will be same:   

(i) In (iv) case 

(ii) In (ii) case  

(iii) In (iii) case 

(iv) In (i) case  

 

Answer

convex-lens-object-at-2f---teachoo.jpg

 

For a convex lens

  • If object is kept at 2F , size of image and object is same
  • Here, 2F = 30 cm
  • So, in option (iii) , size of image and object is same

 

So, the correct answer is (iii)



(d) What is the change in image observed as the object  is moved from infinity towards the concave lens?    

(i) Size of image decreases  

(ii) Size of image becomes highly diminished  

(iii) Size of the image remains unchanged  

(iv) Size of the image increases slightly  

 

Answer

In a convex lens

  • As we move from infinity to Optical center
  • First, image size is a point (object at Infinity)
  • Then, image becomes bigger (object beyond 2F) until i t becomes the size of the objec t (object at 2F)
  • Then image becomes bigger than the object as we move from 2F to F and then from F to O

 

Hence, size of image increases slightly.

 

So, the correct answer is (iv)




(e) Which of the following statement is false for the formation of images by convex lens: 

(i) It forms a real, inverted and diminished image.  

(ii) It forms a virtual, erect and enlarged image.  

(iii) It forms virtual, erect, and diminished images.  

(iv) It forms a real,inverted and enlarged image.  

 

Answer

In a convex lens

  • Real, inverted and diminished image is formed when object is kept beyond 2F.
    So, option (i) is true

  • Virtual, erect and enlarged image is formed when object is kept between F & O.
    So, option (ii) is true

  • Virtual, erect and diminished image cannot be formed.
    So, option (iii) is false

  • Real, inverted and enlarged image is formed when object is kept between F & 2F.
    So, option (iv) is true

 

So, the correct answer is (iii) 

  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