小朋友们安全过马路的口诀大家都耳熟能详吧：“红灯停，绿灯行，黄灯请您等一等”。红绿灯的交通规则对我们是再熟悉不过了。不过，世界上的颜色有千百种，为什么偏偏要用红色来代表“停”呢？

If we are driving a car or crossing a big road, we will stop when the traffic lights turn red. Why do we use red instead of other colors?

As we know, scattering¹ occurs when lights go through the air. For the same media, if the wavelength² is long, the scattering will be short. If the wavelength is short, then the scattering will be long.

Of all of the visible lights, red has the longest wavelength about 1.7 times that of purple, so the scattering is the weakest. That means red light travels far. In fact, it travels farther on rainy or foggy days because of the reduced air transparency.

Using red lights as a stop signal helps drivers in more remote areas see the signal. It helps drivers slow down or brake in time. If drivers didn't see the stop signal until they were close, they may not be able to stop the car in time, possibly causing an accident. Red is also a color that excites people, the optic nerve especially responds to red. In short, red lights help drivers stop in time and help prevent accidents.

Red lights are not only used for stop signals but also warnings. For example, red lights are installed on the tops of tall buildings in cities so that pilots in aircraft can clearly see them at night. Red lights are also used for public security and fire department vehicles.