Question

Power of the lens is -4d, then its focal length is -----?

Answer 1

The power of a lens is a measure of its ability to produce deviation of light. A lens of short focal length produces more deviation of light than that of a lens of larger focal length. So, power of a lens is defined as the reciprocal of its focal length. The unit of power of lens is dioptre, D where f is in meter.

Answer 2

The focal length of lens is -0.25 m.

Step by step Explanation:

Given:

• Power of lens is -4D.

To Find:

• Find the focal length of the lens.

Solution:

Concept to be used:

Focal length of lens is reciprocal of its power.

$\bf\:f = \frac{1}{P}$

here,

f: focal length of the lens in meter.

P: power of lens in Dioptre.

Step 1:

Write the given values.

It is given that, power of lens(P)= -4 D

It is concave lens/Diverging lens.

Step 2:

Find the focal length.

As we know that

$f = \frac{1}{p}$

or

$f = \frac{1}{ - 4}$

or

$\bf \:f = - 0.25 \: m$

Thus,

The focal length of lens is -0.25 m.

Learn more:

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2) calculate the focal length of a convex lens which produces a virtual image at a distance of 50 cm of an object placed 20...

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