Compare wavelength and aperture together

Instrument A uses a clear 8.0 cm circular aperture at 640 nm. Instrument B uses a clear 10.0 cm circular aperture at 400 nm. Both are diffraction-limited under the same point-source criterion.

What is the ratio of B’s Rayleigh angular separation to A’s?

Organize the givens & assumptions

DA = 8.0 cm, λA = 640 nm
DB = 10.0 cm, λB = 400 nm

Comparable mutually incoherent point sources; monochromatic diffraction-limited imaging through clear circular apertures, adequate sampling, no atmospheric or aberration limitation.

A $2$
B $1/2$
C $4/5$
D $5/8$

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