Question

The focal length of a concave lens is 30 cm. what will be its power​

Answer 1

➭ Question :-

$\longrightarrow$The focal length of a concave lens is 30 cm. what will be its power

➭ Given :-

$\longrightarrow$Focal length ,f = +30 or $+ 30 \times {10}^{ - 2}$

➭ To Find :-

$\longrightarrow$ Power,p

➭ Formula required :-

$\longrightarrow$ $\boxed{\sf p = \frac{1}{f}}$

➭ Solution :-

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

$\longrightarrow \rm p = \frac{1}{30 \times {10}^{ - 2} }$

$\longrightarrow \rm p = \cancel \frac{100}{30}$

$\longrightarrow \boxed{ \rm p = 3.3}$

Hence, power P = 3.3dioptre or 3.3 D

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☞ More to know...

• If the image and object are on the same side of the lens, the image distance is negative and the image is virtual.
• For converging mirrors, the focal length is positive.
• Similarly, a converging lens always has a positive f, and a diverging lens has a negative f

Answer 2

$\huge\mathfrak{Answer:}$

$\large\underline{\sf{\blue{Given:}}}$

• We have been given a concave lens
• The focal length of lens is 30 cm

$\large\underline{\sf{\blue{To \: Find:}}}$

• We have to find the power of lens

$\large\underline{\sf{\blue{Concept \: Used:}}}$

Power of a Lens :

Power of lens can be defined as its ability to bent the ray of light according to the nature of lens.

• Convex Lens : Converging Power
• Concave Lens : Diverging Power

Mathematically it can be defined as the reciprocal of focal length

$\boxed{\sf{\green{Power \: (P) = \dfrac{1}{f \: (in \: metre)} }}}$

$\sf{}$

$\large\underline{\sf{\blue{Solution:}}}$

Concave Lens having focal length

• Focal Length ( f ) = 30 cm

Using Proper sign convention

$\boxed{\sf{f = - 30 \: cm \: OR - 0.3 \: m}}$

$\sf{}$

$\large\underline{\mathfrak\red{According \: to \: the \: Question:}}$

Power of lens can be calculated as :

$\longrightarrow \sf{Power = \dfrac{1}{focal \: length } }$

$\longrightarrow \sf{Power = -\dfrac{1}{0.3} }$

$\longrightarrow \sf{P = -\dfrac{10}{3} }$

$\longrightarrow \boxed{\sf{P = - 3.33 \: D} }$

$\sf{}$

Power of given lens is -3.33 Dioptre

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$\large\purple{\underline{\underline{\mathtt{Extra \: Information:}}}}$

• Concave Lens
• A concave lens is lens that posses surface which is curved inside
• It is a diverging lens means it spread the rays of light passing through it
• Focal length and power of a concave lens is negative
• Some uses :
• Concave lens is used to treat short sightedness or myopia
• Also use in telescopes and binoculars

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