**Fog lights** are essential for improving visibility in adverse weather conditions such as **fog, snow, heavy rain, and smoke**. There are two types of fog lights: front and rear. The front lights can be white or yellow, while the rear ones are red.

- **Front fog lights**: They can be used together with position lights, high beam, and low beam. They are useful for improving road visibility in low visibility conditions.

- **Rear fog lights**: These lights are especially important in situations of **poor visibility**, such as during heavy rain. It is mandatory to use them in these conditions so that other drivers can see your vehicle more easily. They can be turned on together with high and low beam lights and front fog lights.

Therefore, in case of **very heavy rain**, it is correct and mandatory to use the rear fog light to ensure road safety.