Solution to 1992 Problem 68

We use the Rayleigh criterion, which states that the minimum angular separation \alpha_0 of two point sources such that ``an observer watching the images on the distant screen can just tell them apart" is, for a circular aperature with diameter D,
\begin{align*}\alpha_0 = \frac{1.22 \lambda}{D}\end{align*}
where \lambda is the wavelength of the light. %(See Bekefi and Barratt pages 578 and 579).
Plugging in the given value for \lambda and \alpha_0, we can find that
\begin{align*}D = \frac{1.22 \lambda}{\alpha_0} = \frac{1.22 \cdot 5500 \cdot 10^{-10} \mbox{ m}}{8 \cdot 10^{-6}} = 0.0839 \...
Therefore, answer (C) is correct.

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