Assuming human pupil to have a radius of 0.25 cm and a comfortable viewing distance of 25cm, the minimum separation between two objects that human eye can resolve at 500 nm wavelength is :

Diameter d=2r=2×0.25=0.5cm, the wavelength of light is λ=500nm=5×10⁻⁷m. We have the formula sinθ=1.22λ/d or sinθ=1.22×5×10⁻⁷/0.5×10⁻²=1.22×10⁻⁴. Since θ is very small, sinθ≈θ, therefore θ=1.22×10⁻⁴ radians. Let x be the minimum separation between two objects. Then x=25cm×θ=25×10⁻²×1.22×10⁻⁴=3.05×10⁻⁵m=30.5μm≈30μm