Question

The focal length of a concave lens of power 2 dipper is: *

+25 cm

-25 cm

+50 cm

-50 cm​

Answer 1

Given:-

• Power ,P = 2D
• Types of lens = Concave lens

To Find:-

• Focal length ,f

Solution:-

According to the Question

We have to calculate the focal length of the concave lens.

As we know that power of lens is defined as the reciprocal of its focal length in metres.

• P = 1/f

where,

• P denote Power
• f denote focal length

Substitute the value we get

$:\implies$ 2 = 1/f

$:\implies$ 2f = 1

$:\implies$ f = 1/2

$:\implies$ f = 0.5 m

$:\implies$ f = 0.5 × 100

$:\implies$ f = 50 cm

Since the focal length of concave lens is always taken as negative .

Therefore,

$:\implies$ f = -50 cm

• Hence, the required option is -50 cm .

Answer 2

Answer:

Given :-

The focal length of a concave lens of power 2 dipper

To Find :-

Focal length

Solution :-

We know that

P = 1/f

2 = 1/f

2 × f = 1

2f = 1

f = 1/2

f = 0.5 m

Converting m into cm

1  m = 100 cm

0.5 m = 0.5 × 100 = 5/10 × 100 = 5 × 10 = 50 cm

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