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  • (a) List four characteristics of the image formed by a concave lens of focal length 20 cm when the object is placed at a distance of  40 cm from its optical centre. (b) The size of image of an object by a convex lens of focal length 20 c

(a) List four characteristics of the image formed by a concave lens of focal length 20 cm when the object is placed at a distance of  40 cm from its optical centre.

(b) The size of image of an object by a convex lens of focal length 20 cm is observed to be reduced to  \frac{1}{3}rd   of its size. Find the distance of the object from the optical centre of the lens.

 

 

 

 
 
 
 
 

Answers (1)

(a)Characteristics of image formed by a cancave lens :

        f=20\; cm\; \; \; \; \; \; \; \mu =-40 \; cm

We know that

        \frac{-1}{f}=\frac{-1}{v}-\frac{1}{\mu }

        \frac{1}{v}=\frac{1}{f}-\frac{1}{\mu }

              =\frac{1}{20}-\frac{1}{40 }=\frac{20}{800}

\frac{1}{v}=\frac{1}{40}

v=40\; cm

(i) image will be formed at 40 cm from optical centre.

(ii) Radius of curvature of lens

        R=-2f

          R=-2\times 20=-40\; cm

(iii) Since image distance is positive therefore image is        virtual.

(iv) Image will be erect.

(v) Image will form opposite side of lens.

(b) Given,

       f=20\; cm\; \; \; \; v=\frac{1}{3}\mu\; cm\; \; \; \; u=?

We know that

        \frac{1}{f}=\frac{1}{v}+\frac{1}{u}

using sign conventions

         \frac{1}{f}=\frac{-1}{u}-\frac{1}{v}

         \frac{1}{20}=\frac{-1}{u}-\frac{3}{4}=\frac{-4}{u}

            u=-4\times 20=80\; cm

              u=-80\; cm

 

Posted by

Sumit Saini

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