Ray Optics

JEE Main level, single correct, medium

209 of 295

Question 209 of 295

An object 2.4 m2.4 \mathrm{~m} in front of a lens forms a sharp image on a film 12 cm12 \mathrm{~cm} behind the lens. A glass plate 1 cm1 \mathrm{~cm} thick, of refractive index 1.501.50 is interposed between lens and film with its plane faces parallel to film. At what distance (from lens) should object be shifted to be in sharp focus on film?

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Answer: (D) 5.6 m5.6 \mathrm{~m}

Case I:u=−240 cm,v=12\mathrm{I}: \mathrm{u}=-240 \mathrm{~cm}, \mathrm{v}=12, by Lens formula
1f=780\frac{1}{f}=\frac{7}{80}
Case II\mathrm{II}: v=12−13=353\mathrm{v}=12-\frac{1}{3}=\frac{35}{3} (normal shift =1−23=13=1-\frac{2}{3}=\frac{1}{3} )
f=780f=\frac{7}{80}
u=5.6u=5.6

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