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The objective lens of a telescope has a diameter of 250 cm. For light of wavelength 600 nm from a distant object, the limit of resolution of the telescope is closest to:

1. 1.5 x 10⁻⁷ rad
2. 2.0 x 10⁻⁷ rad
3. 3.0 x 10⁻⁷ rad
4. 4.5 x 10⁻⁷ rad

Correct answer: 3.0 x 10⁻⁷ rad

Solution

Limit of resolution theta = 1.22*lambda/D. Here lambda = 600 nm = 6x10⁻⁷ m and D = 250 cm = 2.5 m. theta = 1.22*(6x10⁻⁷)/2.5 = 7.32x10⁻⁷/2.5 = 2.93x10⁻⁷ rad, which is about 3.0x10⁻⁷ rad.

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