To solve this problem we will apply trigonometric and optical concepts that allow us to obtain the minimum distance required. The resolution of the eye is given under the following condition,
[tex]\theta = \frac{1.22\lambda}{D}[/tex]
Here,
[tex]\lambda = Wavelength[/tex]
[tex]D = Diameter[/tex]
With the values we have that the diameter will be,
[tex]\theta = \frac{1.22(534nm)}{5.37mm}[/tex]
[tex]\theta = 1.213*10^{-4}[/tex]
The relation between the distance of the lights and the distance from the eye to the lamp is given under the function,
[tex]sin\theta = \frac{d}{L}[/tex]
For small angles [tex]sin\theta = \theta[/tex], then
[tex]\theta = \frac{d}{L}[/tex]
Here,
d = Distance between lights
L = Distance from eye to lamp
[tex]1.213*10^{-4} = \frac{0.673m}{L}[/tex]
[tex]L = \frac{0.673m}{1.213*10^{-4}}[/tex]
[tex]L = 5548.22m[/tex]
Therefore the distance will be 5.5km