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  • Try this! What is the position and nature of image formed by lens combination shown in figure ? (f1, f2are focal lengths )

What is the position and nature of image formed by lens combination shown in figure ? (f1, f2 are focal lengths ) 

  • Option 1)

    40 cm from point B at right; real 

  • Option 2)

          \frac{20}{3}  from point B at right; real 

  • Option 3)

     70 cm from point B at right; real 

  • Option 4)

    70 cm from point B at left; virtual 

Answers (1)

best_answer

 

Thin lens formula -

\frac{1}{v}-\frac{1}{u}= \frac{1}{f}

 

- wherein

u \, and \, v are object and image distance from lens.

For first lens

u_{1}=-20,\, \, \, f_{1}=5cm

so, \frac{1}{v_{1}}-\frac{1}{u_{1}}=\frac{1}{f}

\frac{1}{v}-\frac{1}{-\left ( 20 \right )}=\frac{1}{5}

v_{1}=\frac{20}{3}

So  u_{2}  for 2nd  lens is given by

So  u_{2}=\frac{20}{3}-2=\frac{14}{3}

f_{2}=-5cm

So  \frac{1}{v_{2}}-\frac{1}{u_{2}}=\frac{1}{f_{2}}

\frac{1}{v_{2}}-\frac{1}{\left ( 1_{\frac{4}{3}} \right )}=\frac{1}{-5}

V_{2}=70cm   and image is real

 


Option 1)

40 cm from point B at right; real 

Option 2)

      \frac{20}{3}  from point B at right; real 

Option 3)

 70 cm from point B at right; real 

Option 4)

70 cm from point B at left; virtual 

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