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Physics

The power of a concave lens of focal length 20 cm is :

  1. 5D
  2. 10D
  3. -10D
  4. -5D

Refraction Lens

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Answer

-5D

Reason — As we know,

Power=1focal length\text{Power} = \dfrac{1}{\text{focal length}}

Given,

focal length = 20 cm = 0.2 m

Substituting the value of focal length in formula we get,

Power=10.2 m⇒Power=5 D\text{Power} = \dfrac{1}{\text{0.2 m}} \\[0.5em] \Rightarrow \text{Power} = 5 \text{ D} \\[0.5em]

As the lens used is concave in nature so power is negative.

Hence, Power = – 5D

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