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JEE Advanced 2010Paper 1
Exam
JEE Advanced
Year
2010
Paper
Paper 1
Subject
Physics
Chapter
Ray Optics

JEE Advanced 2010 · Paper 1 · Physics · Ray Optics

The focal length of a thin biconvex lens is 20 ~cm . When an object...

The focal length of a thin biconvex lens is 20 cm20 \mathrm{~cm}. When an object is moved from a distance of 25 cm25 \mathrm{~cm} in front of it to 50 cm50 \mathrm{~cm}, the magnification of its image changes from m25m_{25} to m50m_{50}. The ratio m25m50\frac{m_{25}}{m_{50}} is

Correct answer

6

Step-by-step solution

1v−1u=1f\frac{1}{v}-\frac{1}{u}=\frac{1}{f} or uv−\frac{u}{v}-1=uf\frac{u}{f} or uv=(u+ff)∴m=vu=(fu+f)m25m50=(20−25+20)(20−50+20)=6\begin{array}{rlrl} & \text{ or } & \frac{u}{v} & =\left(\frac{u+f}{f}\right) \\ \therefore & m=\frac{v}{u} & =\left(\frac{f}{u+f}\right) \\ \frac{m_{25}}{m_{50}} & =\frac{\left(\frac{20}{-25+20}\right)}{\left(\frac{20}{-50+20}\right)}=6\end{array}∴\therefore answer is 6 .
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