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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Speed of Light

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Speed of Light

For an experiment using Foucault's apparatus, the setup distances...

For an experiment using Foucault's apparatus, the setup distances are given as follows:
Distance between the rotating and the fixed mirror =16 m= 16 \text{ m},
Distance between the lens and the rotating mirror =6 m= 6 \text{ m},
Distance between the source and the lens =2 m= 2 \text{ m}.
Upon rotating the mirror at a speed of 356356 revolutions per second, a shift of 0.7 mm0.7 \text{ mm} is observed in the image. Determine the speed of light from this data.

Options

  1. A5.97×108 m/s5.97 \times 10^8 \text{ m/s}
  2. B2.98×108 m/s2.98 \times 10^8 \text{ m/s}
  3. C1.49×108 m/s1.49 \times 10^8 \text{ m/s}
  4. D3.00×108 m/s3.00 \times 10^8 \text{ m/s}

Correct answer

B. 2.98×108 m/s2.98 \times 10^8 \text{ m/s}

Step-by-step solution

In Foucault's rotating mirror experiment, the shift in the image is given by the formula:

s=4R2ωac(R+b)s = \dfrac{4 R^2 \omega a}{c (R+b)}

where:
RR = distance between the rotating and fixed mirror = 16 m16 \text{ m}
bb = distance between the lens and the rotating mirror = 6 m6 \text{ m}
aa = distance between the source and the lens = 2 m2 \text{ m}
ω\omega = angular velocity of the rotating mirror = 2πn2 \pi n
nn = frequency of rotation = 356 rev/s356 \text{ rev/s}
ss = shift in the image = 0.7 mm=0.7×10−3 m0.7 \text{ mm} = 0.7 \times 10^{-3} \text{ m}
cc = speed of light

Rearranging the formula to solve for the speed of light cc:

c=4R2(2πn)as(R+b)c = \dfrac{4 R^2 (2 \pi n) a}{s (R+b)}

Substituting the given values and using π≈227\pi \approx \dfrac{22}{7}:

c=8×227×356×(16)2×20.7×10−3×(16+6)c = \dfrac{8 \times \dfrac{22}{7} \times 356 \times (16)^2 \times 2}{0.7 \times 10^{-3} \times (16 + 6)}

c=8×22×356×256×27×0.7×10−3×22c = \dfrac{8 \times 22 \times 356 \times 256 \times 2}{7 \times 0.7 \times 10^{-3} \times 22}

Canceling 2222 from the numerator and denominator:

c=8×356×256×24.9×10−3c = \dfrac{8 \times 356 \times 256 \times 2}{4.9 \times 10^{-3}}

c=4096×3564.9×103c = \dfrac{4096 \times 356}{4.9} \times 10^3

c=14581764.9×103c = \dfrac{1458176}{4.9} \times 10^3

c≈297586×103 m/sc \approx 297586 \times 10^3 \text{ m/s}

c≈2.9758×108 m/sc \approx 2.9758 \times 10^8 \text{ m/s}

Rounding to two decimal places, we get c≈2.98×108 m/sc \approx 2.98 \times 10^8 \text{ m/s}.

Answer: 2.98×108 m/s2.98 \times 10^8 \text{ m/s}
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