If a biconvex lens of material of refractive index 1.5 has focal length 20 cm in air, then its focal length when it is submerged in a medium of refractive index 1.6 is

Option 1 -

–160 cm

Option 2 -

160 cm

Option 3 -

1.6 cm

Option 4 -

–16 cm

2 Views | Posted 2 days ago

Asked by Shiksha User

1 Answer

Answered by

alok kumar singh | Contributor-Level 10

2 days ago

Correct Option - 1

Detailed Solution:

$\frac{1}{20} = (1.5 - 1) (\frac{2}{R})$

R = 20cm

$\frac{1}{f'} = (\frac{1.5}{1.6} - 1) (\frac{2}{R})$

$= \frac{- 1}{16} \times \frac{2}{20}$

f' = – 160cm

<!- [if gte mso 9]><xml> <o:OLEObject Type="Embed" ProgID="Equation.DSMT4" ShapeID="_x0000_i1025" DrawAspect="Content" ObjectID="_1820661991"> </o:OLEObject></xml><! [endif]-> <math> <mrow> <mfrac> <mrow> <mn>1</mn> </mrow> <mrow> <mn>2</mn> <mn>0</mn> </mrow> </mfrac> <mo>=</mo> <mrow> <mo> (</mo> <mrow> <mn>1</mn> <mo>.</mo> <mn>5</mn> <mo>−</mo> <mn>1</mn> </mrow> <mo>)</mo> </mrow> <mrow> <mo> (</mo> <mrow> <mfrac> <mrow> <mn>2</mn> </mrow> <mrow> <mi>R</mi> </mrow> </mfrac> </mrow> <mo>)</mo> </mrow> </mrow> </math> R = 20cm <math> <mrow> <mfrac> <mrow> <mn>1</mn> </mrow> <mrow> <mi>f</mi> <mo>'</mo> </mrow> </mfrac> <mo>=</mo> <mrow> <mo> (</mo> <mrow> <mfrac> <mrow> <mn>1</mn> <mo>.</mo> <mn>5</mn> </mrow> <mrow> <mn>1</mn> <mo>.</mo> <mn>6</mn> </mrow> </mfrac> <mo>−</mo> <mn>1</mn> </mrow> <mo>)</mo> </mrow> <mrow> <mo> (</mo> <mrow> <mfrac> <mrow> <mn>2</mn> </mrow> <mrow> <mi>R</mi> </mrow> </mfrac> </mrow> <mo>)</mo> </mrow> </mrow> </math>            <!- [if gte mso 9]><xml> <o:OLEObject Type="Embed" ProgID="Equation.DSMT4" ShapeID="_x0000_i1026" DrawAspect="Content" ObjectID="_1820661993"> </o:OLEObject></xml><! [endif]-> <math> <mrow> <mo>=</mo> <mfrac> <mrow> <mo>−</mo> <mn>1</mn> </mrow> <mrow> <mn>1</mn> <mn>6</mn> </mrow> </mfrac> <mo>×</mo> <mfrac> <mrow> <mn>2</mn> </mrow> <mrow> <mn>2</mn> <mn>0</mn> </mrow> </mfrac> </mrow> </math>                  <!- [if gte mso 9]><xml> <o:OLEObject Type="Embed" ProgID="Equation.DSMT4" ShapeID="_x0000_i1027" DrawAspect="Content" ObjectID="_1820661994"> </o:OLEObject></xml><! [endif]->f' = – 160cm

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If a biconvex lens of material of refractive index 1.5 has focal length 20 cm in air, then its focal length when it is submerged in a medium of refractive index 1.6 is

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