Focal length of a concave lens is 400 cm- then power of the lens is-A- 0-5 DB- 1 DC- 1-5 DD- -0-25 D

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Power of a Lens

Focal length ...

Question

Focal length of a concave lens is 400 cm, then power of the lens is:

$0.5 D$

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$1 D$

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$1.5 D$

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$- 0.25 D$

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Solution

The correct option is D
$- 0.25 D$

Concave lens is a diverging lens and its focal length is negative.

Given: Focal length of concave lens $f = - 400 c m = \frac{- 400}{100} = - 4 m$

We know, power of a lens $P = \frac{1}{f}$

$\Rightarrow$ $P = \frac{1}{- 4} = - 0.25 D$

Therefore, power of the given lens is - 0.25 D.

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Q. What will be the power of a concave lens whose focal length is

2 m

Focal length of a concave lens is 25 cm. Calculate its power.

Question 8
Which of the following statements is true?
(a) A convex lens has 4 dioptre power having a focal length 0.25 m.
(b) A convex lens has -4 dioptre power having a focal length -0.25 m.
(c) A concave lens has 4 dioptre power having a focal length 0.25 m.
(d) A concave lens has 4 dioptre power having a focal length -0.25 m.

Q. If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :
(a) −4 cm
(b) −400 mm
(c) −4 m
(d) −40 cm

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Focal length of a concave lens is 400 cm, then power of the lens is:

The correct option is D −0.25 D Concave lens is a diverging lens and its focal length is negative. Given: Focal length of concave lens f=−400 cm=−400100=−4 m We know, power of a lens P=1f ⇒P=1−4=−0.25 D Therefore, power of the given lens is - 0.25 D.