Late one night on a highway, a car speeds by you and fades into the distance. Under these conditions the pupils of your eyes have diameters of about 7.1 mm. The taillights of this car are separated by a distance of 1.4 m and emit red light (wavelength = 644 nm in vacuum). How far away from you is this car when its taillights appear to merge into a single spot of light because of the effects of diffraction?