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JEE Main 2021 (26 Aug, Shift 1) · Reflection & Mirrors

Reflection & Mirrors question from JEE Main 2021 (26 Aug, Shift 1)

Q34JEE Main 2021 · 26 AUG · SHIFT 1Medium

Reflection & Mirrors

Car B overtakes another car A at a relative speed of 40 ms−1{}^{-1}. How fast will the image of car B appear to move in the mirror of focal length 10 cm fitted in car A, when the car B is 1.9 m away from the car A?

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Answer: (d)

  1. The rear-view mirror is convex: f=+0.1 mf = +0.1\text{ m}, u=−1.9 mu = -1.9\text{ m}.
  2. m=ff−u=0.10.1+1.9=120m = \frac{f}{f - u} = \frac{0.1}{0.1 + 1.9} = \frac{1}{20}
  3. Image speed vI=m2vO=1400×40=0.1 m/sv_I = m^2 v_O = \frac{1}{400} \times 40 = 0.1\text{ m/s}
  4. So the answer is (d) 0.1 m s−1^{-1}.
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