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Wave Optics question

2020 · 4 Sep · Shift 1 · Q57
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Wave Optics question

2020 · 4 Sep · Shift 1 · Q57

JEE MainPhysicsWave OpticsMCQ+4 / −1
A beam of plane polarised light of large cross-sectional area and uniform intensity of 3.3 Wm-2 falls normally on a polariser (cross sectional area 3 ×\times× 10-4 m2) which rotates about its axis with an angular speed of 31.4 rad/s. The energy of light passing through the polariser per revolution, is close to :
  1. A
    1.0 ×\times× 10-5 J
  2. B
    1.0 ×\times× 10-4 J
  3. C
    1.5 ×\times× 10-4 J
  4. D
    5.0 ×\times× 10-4 J
View written solutionFree

Correct answer: B

  1. Given data
  • Intensity of incident plane polarised light: I0=3.3 W m−2I_0 = 3.3\ \text{W m}^{-2}I0​=3.3 W m−2
  • Area of polariser: A=3×10−4 m2A = 3 \times 10^{-4}\ \text{m}^2A=3×10−4 m2
  • Angular speed of rotation: ω=31.4 rad s−1\omega = 31.4\ \text{rad s}^{-1}ω=31.4 rad s−1

We need the energy transmitted in one complete revolution.


  1. Instantaneous transmitted intensity

Since the incident light is plane polarised and the polariser rotates, the transmitted intensity is given by Malus' law:

I=I0cos⁡2θI = I_0 \cos^2 \thetaI=I0​cos2θ

where θ=ωt\theta = \omega tθ=ωt

So,

I(t)=I0cos⁡2(ωt)I(t) = I_0 \cos^2(\omega t)I(t)=I0​cos2(ωt)


  1. Instantaneous transmitted power

Power transmitted through the polariser is

P(t)=I(t)A=I0Acos⁡2(ωt)P(t) = I(t) A = I_0 A \cos^2(\omega t)P(t)=I(t)A=I0​Acos2(ωt)

Substitute values:

P(t)=3.3×3×10−4cos⁡2(ωt)P(t) = 3.3 \times 3\times 10^{-4} \cos^2(\omega t)P(t)=3.3×3×10−4cos2(ωt)

P(t)=9.9×10−4cos⁡2(ωt) WP(t) = 9.9 \times 10^{-4} \cos^2(\omega t)\ \text{W}P(t)=9.9×10−4cos2(ωt) W


  1. Time for one revolution

For one full revolution,

T=2πω=2π31.4≈0.2 sT = \frac{2\pi}{\omega} = \frac{2\pi}{31.4} \approx 0.2\ \text{s}T=ω2π​=31.42π​≈0.2 s


  1. Energy transmitted in one revolution

Energy is

E=∫0TP(t) dt=I0A∫0Tcos⁡2(ωt) dtE = \int_0^T P(t)\,dt = I_0 A \int_0^T \cos^2(\omega t)\,dtE=∫0T​P(t)dt=I0​A∫0T​cos2(ωt)dt

Over one full cycle, average value of cos⁡2\cos^2cos2 is

⟨cos⁡2θ⟩=12\langle \cos^2 \theta \rangle = \frac{1}{2}⟨cos2θ⟩=21​

Therefore,

E=I0A⋅T⋅12E = I_0 A \cdot T \cdot \frac{1}{2}E=I0​A⋅T⋅21​

Substitute:

E=3.3×3×10−4×0.2×12E = 3.3 \times 3\times 10^{-4} \times 0.2 \times \frac{1}{2}E=3.3×3×10−4×0.2×21​

E=9.9×10−4×0.1E = 9.9\times 10^{-4} \times 0.1E=9.9×10−4×0.1

E=9.9×10−5 JE = 9.9 \times 10^{-5}\ \text{J}E=9.9×10−5 J

This is approximately

E≈1.0×10−4 JE \approx 1.0 \times 10^{-4}\ \text{J}E≈1.0×10−4 J


  1. Option matching

The closest option is:

B: 1.0×10−4 J1.0 \times 10^{-4}\ \text{J}1.0×10−4 J


  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So they agree.

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