If a car is 500 m ahead of you, you view the tail lights as one long, red light. If the car is 100 m ahead of you, you can observe that the tail lights are in reality several red lights placed close to each other. This is since..
i) The pupil of your eye consists of a finite size
ii) The Doppler Effect for light shifts the frequency of tail lights.
iii) Light disperses in the lens of your eye.
iv) Light is made up of the photons.