The backlights of vehicles are red. Not just the backlights of vehicle but most of the light signals that signify danger are red. Is the red color of backlights of vehicles arbitrary? Or, Is our eye too sensitive to a red light?

Photo by Jesse Collins on Unsplash

How Sensitive To Red Light Is Our Eye?

Let’s analyze the sensitiveness of our eye towards red light. Our eyes are usually less sensitive to red light. You might have thought the other way round but it’s not. Although red light is mostly used in hoarding boards and different signals, our eyes are almost insensitive towards it during dim light.

Our eyes are most sensitive to yellowish-green light during bright light while the sensitivity is maximum for blue light during dim light. This makes us unable to make a clear distinction of reddish colors during a dim light. Also, during bright light, the red color does not show much sensitivity.

But, our aim is not to explain the use of red light-dependent upon such sensitiveness. Our major concern here is to understand the intensity variation of fixed color source and it’s not studying the color of surrounding infused with various colors of light. Whatever the sensitivity maybe we can see red light clearly due to lack of its scattering.

What is the Reason Behind the Red Color of Back Lights?

Red means fury, red means danger, and red means warning. The purpose red color of backlights is to warn the driver behind a vehicle while the vehicle is slowing down or stopping.

It could have been another color too but early manufacturers used red light instead. Was it necessary to use red light over other colors? Greenlight could have made it meaningful because it is more sensitive but why red?

The scattering of light is inversely proportional to the fourth power of wavelength. Among the visible lights, red light has the longest wavelength. Due to this, it does not scatter much as other visible lights. It is the color of light that is scattered least, while blue, indigo and violet are scattered the most.

The least scattering nature of red light can enable it to reach long-distance without dropping intensity. So, the red color of backlights can travel longer distances. Even during foggy weather, red light is more likely to be seen for farther distance in comparison to other visible light. So, the red color of backlights can give warnings and information to a person very far away.

So, the red color of backlight is scientific, not arbitrary. It has significantly contributed to decreasing the number of road accidents.

Why is Red Light Used as a Warning Sign?

Red lights are used in a variety of places, especially when the light has to give warnings. The backlight warns the drivers of other vehicles behind a vehicle about the application of brakes. Also, the warning lights at the top of the towers and lights at danger sign is red.

As stated above, red light can be seen for farther distance. So, it is the best possible danger signal. The skyscrapers also have red light as a warning sign on their top. This can warn airplanes about the obstacle from far away and they can easily divert their path. The use of blue light could make it difficult to be seen for farther distance.

The hoarding boards glowing with red light can be seen for a longer distance and can be a better advertisement alternative. So, remember to put on a red glowing hoarding board if you are trying to advertise anything. Red color doesn’t just make things attractive, it also saves us from different accidents and mishaps by warning us.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.