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A telescope objective of diameter 200 cm is used to observe light of wavelength 500 nm from a star. Calculate the telescope's limit of resolution (smallest resolvable angle).

1. 305 * 10⁻⁹ radian
2. 152.5 * 10⁻⁹ radian
3. 610 * 10⁻⁹ radian
4. 457.5 * 10⁻⁹ radian

Correct answer: 305 * 10⁻⁹ radian

Solution

By the Rayleigh criterion, the angular limit of resolution of a circular aperture is 1.22*lambda/D. With lambda = 500 nm and D = 200 cm = 2 m, the result is about 305 nanoradian.

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