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Geometrical Optics question

2018 · Q117
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Geometrical Optics question

2018 · Q117

NEETPhysicsGeometrical OpticsMCQ+4 / −1
An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be
  1. A
    30 cm away from the mirror
  2. B
    36 cm away from the mirror
  3. C
    30 cm towards the mirror
  4. D
    36 cm towards the mirror
View written solutionFree

Correct answer: B

NEET 2018 Physics - Geometrical Optics Question 25 English Explanation

Using mirror formula,

$${1 \over f} = {1 \over {{v_1}}} + {1 \over {{u_1}}}$$

$${1 \over { - 15}} = {1 \over {{v_1}}} - {1 \over {40}}$$

$ \Rightarrow $ v1 = –24 cm

When object is displaced by 20 cm towards mirror.

Here u2 = –20

So, $${1 \over f} = {1 \over {{v_2}}} + {1 \over {{u_2}}}$$

$${1 \over { - 15}} = {1 \over {{v_2}}} - {1 \over {20}}$$

$ \Rightarrow $ v2 = -60 cm

So, the image will be shift away from mirror by (60 – 24) cm = 36 cm.
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