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A convex lens of focal length 20 cm is placed glass slab of refractive index $\large\frac{3}{2}$.A parallel beam of light is incident on the convex lens. The light will converge.

(a)at a distance of 15 cm from lens

(b)at a distance of 25 cm from lens

(c)at a distance of 20 cm from lens

(d)at a distance of 30 cm from lens

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1 Answer

The parallel rays will converge 20 cm from the lens if no glass slab is present $OI_1=20.$
$\therefore PI_1=20-10=10 \;cm$
But $PI_2=\mu P I_1$
$PI_2=\large\frac{3}{2} $$\times 10 =15 \;cm$
$\therefore OI_1 =15+10 =25\;cm$
Hence d is the correct answer.
answered Jan 22, 2014 by meena.p
 

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