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Geometrical Optics question

2020 · 5 Sep · Shift 1 · Q53
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  5. /2020 · 5 Sep · Shift 1 · Q53

Geometrical Optics question

2020 · 5 Sep · Shift 1 · Q53

JEE MainPhysicsGeometrical OpticsMCQ+4 / −1
For a concave lens of focal length f, the relation between object and image distances u and v, respectively, from its pole can best be represented by (u = v is the reference line) :
  1. A
    JEE Main 2020 (Online) 5th September Morning Slot Physics - Geometrical Optics Question 161 English Option 1
  2. B
    JEE Main 2020 (Online) 5th September Morning Slot Physics - Geometrical Optics Question 161 English Option 2
  3. C
    JEE Main 2020 (Online) 5th September Morning Slot Physics - Geometrical Optics Question 161 English Option 3
  4. D
    JEE Main 2020 (Online) 5th September Morning Slot Physics - Geometrical Optics Question 161 English Option 4
View written solutionFree

Correct answer: D

  1. Use the lens formula

For a thin lens,

1v−1u=1f.\frac{1}{v}-\frac{1}{u}=\frac{1}{f}.v1​−u1​=f1​.

For a concave lens, the focal length is negative. Let its magnitude be f>0f>0f>0, so actual focal length is −f-f−f. Hence,

1v−1u=−1f.\frac{1}{v}-\frac{1}{u}=-\frac{1}{f}.v1​−u1​=−f1​.
  1. Rearrange to get relation between uuu and vvv

Multiply by uvfuvfuvf:

f(u−v)=−uv.f(u-v)=-uv.f(u−v)=−uv.

So,

uv=f(v−u).uv=f(v-u).uv=f(v−u).

This is the required relation between uuu and vvv.

  1. Nature of image for a concave lens

For a real object placed to the left of a concave lens:

  • u<0u<0u<0
  • image is virtual and on the same side, so v<0v<0v<0

Thus the physically relevant curve lies in the third quadrant of the uuu-vvv plane.

  1. Check asymptotic/limiting behavior

From

1v=1u−1f,\frac{1}{v}=\frac{1}{u}-\frac{1}{f},v1​=u1​−f1​,

we get

v=uff−u.v=\frac{uf}{f-u}.v=f−uuf​.

Now examine limits:

  • As u→0−u\to 0^-u→0−, v→0−.v\to 0^-.v→0−.
  • As u→−∞u\to -\inftyu→−∞, v→−f.v\to -f.v→−f.

So the graph starts near the origin in the third quadrant and approaches the horizontal line v=−fv=-fv=−f as u→−∞u\to -\inftyu→−∞.

  1. Compare with the reference line u=vu=vu=v

For a concave lens, the image always lies between optical center and focus, so ∣v∣<∣u∣.|v|<|u|.∣v∣<∣u∣. Since both uuu and vvv are negative, this means v>u.v>u.v>u. Therefore the curve lies above the line u=vu=vu=v in the third quadrant.

  1. Hence the correct graph

The correct option must show:

  • branch in the third quadrant,
  • passing near the origin,
  • lying above the line u=vu=vu=v,
  • approaching v=−fv=-fv=−f for large negative uuu.

This corresponds to Option D.

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