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Q: The power of a lens is –5 D. Its focal length is :
  • A. –50 cm
  • B. –20 cm
  • C. 20 cm
  • D. 50 cm
Correct Answer: Option B - Given, Power (P) = –5D As we know that, Power = 1/Focal Length(in metres) Power = 100/Focal Length(in centimetres) focal length (f) = 100/–5 = –20 cm
B. Given, Power (P) = –5D As we know that, Power = 1/Focal Length(in metres) Power = 100/Focal Length(in centimetres) focal length (f) = 100/–5 = –20 cm

Explanations:

Given, Power (P) = –5D As we know that, Power = 1/Focal Length(in metres) Power = 100/Focal Length(in centimetres) focal length (f) = 100/–5 = –20 cm