The law states you dim your lights when the approaching vehicle is within 500ft or if you are following a vehicle within 500 ft. It's in the commercial drivers handbook from the DMV. I assume the same info is available in the non commercial handbook but I don't have one to check.
Your low beam lights should be on when driving after sunset (twilight) and when it is raining. Some states require headlights or daytime running lights to be on in various conditions or at various times.You should not use your high-beam lights when you are closely behind another vehicle, or when approaching another vehicle head-on on a two-way road.
500 feet
low beam
Low beams
low beam
Low beam headlights and fog lights if the vehicle has fog lights.
Never!
Low beams
Fog Lights only, Low Beam only or Fog Lights + Low Beams.
this might be refraction
Low beam or fog lamps only.
Bulbs blew