Two stars are 10 light years away from the earth. They are seen through a telescope of objective diameter 30 cm. The wavelength of light is 600 nm. To see the stars just resolved by the telescope, the minimum distance between them should be (1 light year=9.46×10¹⁵m) of the order of:

We have, Δθ=0.61λ/D
where, λ = wavelength of light = 600 nm = 600 × 10⁻⁹ m
D = diameter of the objective = 30 cm = 0.3 m
Δθ = angular separation between the stars
The distance between the stars is given by:
l = RΔθ
where, R = distance of the stars from the earth = 10 light years = 10 × 9.46 × 10¹⁵ m = 9.46 × 10¹⁶ m
Therefore, l = R × (0.61λ/D) = (9.46 × 10¹⁶ m) × (0.61 × 600 × 10⁻⁹ m) / 0.3 m
l = 1.15 × 10¹¹ m = 1.15 × 10⁸ km