Question

A converging lens has a focal length of 25 cm. Calculate its power and express it according to sign convention. ​

Answer 1

Answer :

• power of given Converging lens is +4 Dioptre.

Explanation :

given that,

• focal length of converging lens is 25 cm; so,

         f = + 25 cm

( because focal length of converging lens is always positive )

we need to find

• power of the converging lens

        P = ?

Now,

As we know Power is known as the reciprocal of focal length in metres.

• P = 1 / f

[ where P is power of lens in Dioptre unit and f is focal length of lens in metres unit ]

so,

Converting focal length of given lens from cm unit to metre unit

→ f = 25 cm

→ f = 25 / 100  m

→ f = 0.25 m

Calculating power of the given lens

→ P = 1 / f

→ P = 1 / (0.25)

→ P = + 4 D

therefore,

• Power of the converging lens of focal length 25 cm is +4 Dioptre.

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• Power of the lens is defined as the degree of convergence or divergence of lens.
• Optical power is also known as the reciprocal of focal length of lens.
• SI unit of optical power : Dioptre (D)
• 1 Dioptre =  m⁻¹
• Power = 1 / (focal length of lens)
• Dimension of optical power : [ kg⁰ s⁰ m⁻¹ ]

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Answer 2

$\mathtt{\huge{\underline{\red{Question\:?}}}}$

✴ A converging lens has a focal length of 25 cm. Calculate its power and express it according to sign convention.

$\mathtt{\huge{\underline{\green{Answer:-}}}}$

✒ It's Power according to the sign convention rule is +4 dioptre.

$\mathtt{\huge{\underline{\pink{Solution:-}}}}$

Given :-

• A converging lens.

• A focal length ( f ) 25 cm = 0.25 m.

To Find :-

• Its power and express it according to sign convention.

Calculation :-

A converging lens is also known as a convex lens .

• It converts parallel rays of light to convergent rays and produces a real image.

• The object is away from the focal point .

• The image formed is real and inverted.

We have our,

Focal length ( f ) as 0.25 m .

We know that the formulae of power of lens is

P = $\dfrac{1}{f}$

Putting value,

➡ P = $\dfrac{1}{f}$

➡ P = $\dfrac{1}{0.25}$

➡ P = 4 D

S.I. unit of power is Dioptre (D) .

The sign convention for this is +4 D.

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