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The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between the lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?

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As the image is obtained on the screen , it is real . 


Magnification , m = –3 , 

v = 80 cm 

u = ? 

As m = v/u 

so, –3 = 80/u, 

u = –80/3 cm .


1/f = 1/v - 1/u 

=1/80 + 3/80 

= 4/80 

= 1/20 

1/f = 1/20cm 

so, f = 20 cm . 

The lens is convex and image formed at 80 cm from the lens is real and inverted.

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