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CBSE Class 10 Science 2017 Outside Delhi Term II Set 1 Paper

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Question : 16 of 36
Marks: +1, -0
If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 D10\,\mathrm{D}, what is its focal length in the Cartesian system ?
Solution:  
It is a concave lens (diverging lens).
Power of a lens is given by the relation
P=1f(  in metre  )P = \frac{1}{f(\,\text{ in metre }\,)}
∵P=10 D\because P = 10\,\mathrm{D}
∴f=1P=110=0.1 m\therefore f = \frac{1}{P} = \frac{1}{10} = 0.1\,\mathrm{m}
or −10 cm-10\,\mathrm{cm}, as lens is concave lens.
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