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An Image Formed on a Screen is Three Times the Size of the Object. the Object and Screen Are 80 Cm Apart When the Image is Sharply Focussed. Calculate Focal Length of the Lens. - Science

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed. 

 Calculate focal length of the lens.

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Magnification (m) = 3
Object distance (u) = ?
Image distance (v) = ?
Focal length (f) = ?
Distance between image and object (v + u) = 80 cm
v = 80-u
We know that: 


`or 3=(80-u)/u` 

`or 3u=80-u` 

`or 4u=80` 

`or u=20cm` 

thus, v=80-u 

=80-20=60 cm 

Putting these values in lens formula, we get: 




`f=60/4=15 cm`

  Is there an error in this question or solution?
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Lakhmir Singh Class 10 Physics (Science)
Chapter 5 Refraction of Light
Q 20.2 | Page 257
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