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A student obtained a sharp image of a candle flame  placed at the distant end of the laboratory table  on a screen using a concave mirror to determine  its focal length. The teacher suggested that he  focus on a distant building about 1 km away from the  laboratory, for getting a more correct value of the  focal length. In order to focus the distant building  on the same screen the student should slightly move the:  

(a) mirror away from the screen  

(b) screen away from the mirror  

(c) screen towards the mirror  

(d) screen towards the building  




In case of Concave mirror , here, 

  • Initially, the student is taking an image of object kept beyond C
  • When an object is kept beyond C, image is formed between C & F.
  • So, initially, screen is kept between C & F
  • But when we focus the distant building 1km away, the object is kept at infinity
  • So, image formed will be at F
  • Thus, we move the screen from between C&F to F, 
  • i.e. We move screen towards the mirror


Obtaining Sharp image of Candle on the screen.jpg

So, the correct answer is (c) 

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CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Accounts and English at Teachoo