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  • How far should an object be placed from a convex lens of focal length to obtain its real image at a distance of <img alt="30\; cm" src="https://entrance

How far should an object be placed from a convex lens of focal length 20\; cm to obtain its real image at a distance of 30\; cm from the lens ? Determine the height of the image if the object is 4\; cm tall.

 

 

 

 
 
 
 
 

Answers (1)

Convex lens -

Focal length =20\; cm=f

Image distance =30\; cm=v

Object distance =?\; =u

Height of object=4\; cm =h

From lens formula

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

    \frac{1}{u}=\frac{1}{v}-\frac{1}{f}=\frac{1}{30}-\frac{1}{20}=\frac{-1}{60}

    u=-60\; cm

From magnification formula

    m=\frac{v}{u}=\frac{30}{-60}=\frac{-1}{2}

    m=\frac{{h}'}{h}=\frac{-1}{2}

                {h}'=\frac{-1}{2}\times 4

                {h}'=-2\; cm

Weight of image =-2\; cm

    

Posted by

Sumit Saini

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