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Question #43738

Difficulty: 8/10 | 🟢 Free

The Question

A uniform cylindrical telescope of mass M=10 kgM = 10 \text{ kg} and length L=0.6 mL = 0.6 \text{ m} is suspended vertically from a frictionless pivot at its upper end in a uniform gravitational field g=10 m/s2g = 10 \text{ m/s}^2. The telescope has an objective lens of diameter D=6.1 cmD = 6.1 \text{ cm} and observes light of wavelength λ=500 nm\lambda = 500 \text{ nm}. A horizontal impulse J=1.0×10−4 N sJ = 1.0 \times 10^{-4} \text{ N s} is applied to the lower end of the telescope, causing it to oscillate as a physical pendulum. A long-exposure photograph of a distant binary star system is taken. Due to the oscillation, the time-averaged image of each star is broadened into a line segment. Assume the effective angular width of a single star's image is W=Δθmotion+ΔθRW = \Delta \theta_{\text{motion}} + \Delta \theta_R, where Δθmotion\Delta \theta_{\text{motion}} is the peak-to-peak angular sweep of the telescope's optical axis, and ΔθR=1.22λD\Delta \theta_R = 1.22 \frac{\lambda}{D} is the static Rayleigh resolution limit. The binary stars are considered just resolved when their actual angular separation Δϕ\Delta \phi equals WW. Calculate the minimum angular separation Δϕ\Delta \phi required to resolve the binary star system.

Options

  • A.

    3.0×10−5 rad3.0 \times 10^{-5} \text{ rad}

  • B.

    2.0×10−5 rad2.0 \times 10^{-5} \text{ rad}

  • C.

    5.0×10−5 rad5.0 \times 10^{-5} \text{ rad}

  • D.

    5×10−5 rad\sqrt{5} \times 10^{-5} \text{ rad}

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Explanation

How the Examiner Thinks

Intuition_Breaker

Train your brain to resist "obvious" answers that are designed to deceive your intuition.

Concepts Behind This Question

Angular Resolution Limit|Physical Pendulum Inertia|Small Angle Approximation

Why This Question Is Tricky

Common Mistakes

This question tests the synthesis of rigid body dynamics (physical pendulum) with wave optics (Rayleigh criterion).

The primary trap lies in the 'Oscillatory Aperture Modulation' concept: students often fail to recognize that the motion blur of an oscillating system spans the peak-to-peak amplitude (2θ02\theta_0), not just the single-sided amplitude (θ0\theta_0).

Additionally, students may incorrectly calculate the moment of inertia by using the center-of-mass formula (ML2/12ML^2/12) instead of the parallel axis theorem for a pivot at the end (ML2/3ML^2/3), or they may incorrectly add the linear motion blur and diffraction limits in quadrature rather than linearly as explicitly defined by the problem's convolution model.

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Question Information

SubjectPhysics
ChapterWave Optics - Class 12
ConceptAngular Resolution Limit
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree