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Q: Which of the following statements are correct? (A) For a lens the first principal focus is the position of the object whose image is at infinity (B) For a lens the first principal focus is the position of the object whose image is formed at twice the focal length. (C) For a lens the second principal focus is the real image whose object is at infinity (D) For a lens the second principal focus is the position of the object whose image is at twice the focal length
  • A. Both (B) and (C)
  • B. Both (B) and (D)
  • C. Both (A) and (C)
  • D. Both (A) and (B)
Correct Answer: Option C - Primary or Principal focus is defined as the point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity and oppositely, if the parallel light says coming from infinity after converging from the lens get focus on the second principal focus. Therefore both (a) and (c) are correct.
C. Primary or Principal focus is defined as the point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity and oppositely, if the parallel light says coming from infinity after converging from the lens get focus on the second principal focus. Therefore both (a) and (c) are correct.

Explanations:

Primary or Principal focus is defined as the point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity and oppositely, if the parallel light says coming from infinity after converging from the lens get focus on the second principal focus. Therefore both (a) and (c) are correct.