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A student-focussed the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle screen and the lens as under 

the Position of candle = 12.0 cm 

Position of convex lens = 50.0 cm 

Position of the screen = 88.0 cm 

(i) What is the focal length of the convex lens? 

(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm? 

(iii) What will be the nature of the image formed if he further shifts the candle towards the lens? 

(iv) Draw a ray diagram to show the formation of the image in case (iii) as said above.

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Position of the candle flame = 12.0 cm 

Position of the lens= 50.0 cm 

Position of the screen=88.0 cm 

(i) u= 50-12= 38 cm. 

Image distance v = 88 - 50 = 38cm

Focal length =

 1/ v - 1/u = 1/f

f = 19cm

(ii) Object distance u = 50 - 31 = 19 cm 


Object distance = focal length 

Hence the I mage is formed at infinity.

(iii) If he further shifts the candle towards the lens. 

  • The object comes between F and 0. In this case. 
  • Image is virtual, enlarged and erect and is formed on the same side of lens.


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