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Why Are Some Headlights Blue? High Intensity Discharge Lamps

Have you seen cars with headlights that appear blue?

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Photo: Travy (flickr)

Some cars have high intensity discharge lamp headlights that appear to be greenish-blue

Have you ever been driving along a country road at night, and all of a sudden a car comes around the bend and shines their high beams right into your eyes? Ugh, they’re so bright!

High Intensity Discharge Lamps

You also notice that their lights look a bit bluer than you’re used to. That’s because they might be. Regular headlights are made up of white light, which is a combination of all colors of the rainbow.

But these blue lights, while still containing a mixture of all colors, contain more blue wavelengths, making them appear more blue to our eyes.

Many luxury cars have these types of lights, called High Intensity Discharge lamps, because the headlights seem brighter under most conditions.

What Happens In Fog?

They actually make visibility worse.

Fog is made up of suspended water particles in the air, like a cloud settled down on the earth. But blue light scatters more effectively off these water particles than regular headlights do.

This is because the shorter the wavelengths of light, the better it scatters off of air and water particles.

Blue Light

So blue light, which has very short wavelengths, is much more likely to scatter than the longer wavelengths, like red light.

This means that when blue light hits the water particles, it’s more likely bounce off, going back into your eyes, and reducing your ability to see through the fog.

It also increases the glare for everyone, since blue light scatters better inside our eyes as well. In the regular headlights, there are still a fair amount of smaller wavelengths, so it scatters too. But since it also has more of the longer wavelengths, more light gets through the fog. So you can still see–sort of.

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