The **Rayleigh criterion** for the resolution of two point sources by a telescope states that two sources are just resolved when:
ATheir angular separation equals $\lambda/D$
BTheir angular separation equals $1.22 \lambda/D$ (where $D$ is aperture diameter)
CTheir angular separation is zero
DTheir angular separation equals $D/\lambda$
Answer & Solution
Correct answer: B. Their angular separation equals $1.22 \lambda/D$ (where $D$ is aperture diameter)
Rayleigh: the first dark Airy ring of one source must coincide with the central maximum of the other. For a circular aperture of diameter $D$, this gives minimum angular separation $\theta = 1.22\lambda/D$. Resolving power $R = 1/\theta = D/(1.22\lambda)$.
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