小朋友們安全過馬路的口訣大家都耳熟能詳吧：“紅燈停，綠燈行，黃燈請您等一等”。紅綠燈的交通規則對我們是再熟悉不過了。不過，世界上的顏色有千百種，為什么偏偏要用紅色來代表“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, scattering occurs when lights go through the air. For the same media, if the wavelength 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.

（北京林業大學通訊員黃典鈺供稿 英語點津 Annabel 編輯）