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Author Topic: Why are fog lights yellow?  (Read 24179 times)

paul.fr

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Why are fog lights yellow?
« on: 18/02/2008 16:44:55 »
What's so special about yellow? Could they be any other colour, and be as effective?


 

Offline DoctorBeaver

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Why are fog lights yellow?
« Reply #1 on: 18/02/2008 19:07:26 »
Could it be something to do with the wavelength of yellow light not being diffracted so much by the water droplets?
 

another_someone

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Why are fog lights yellow?
« Reply #2 on: 18/02/2008 19:56:20 »
I don't think it is diffraction as such.  Water absorbs blue/green light, so yellow light will be less absorbed by the water.
 

Offline DoctorBeaver

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Why are fog lights yellow?
« Reply #3 on: 18/02/2008 22:00:43 »
That makes sense
 

lyner

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Why are fog lights yellow?
« Reply #4 on: 18/02/2008 23:14:27 »
I think it was thought to reduce back-scatter; red would be even better but you would, perhaps, think some guy was reversing at you at 60mph if he had red headlights. Scary.
 

another_someone

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Why are fog lights yellow?
« Reply #5 on: 19/02/2008 00:01:36 »
I think it was thought to reduce back-scatter; red would be even better but you would, perhaps, think some guy was reversing at you at 60mph if he had red headlights. Scary.

I suspect the other advantage may be that our eyes are probably more efficient at yellow than red.
 

lyner

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Why are fog lights yellow?
« Reply #6 on: 19/02/2008 09:22:56 »
Sounds right, too.
The 'Continentals' had a thing about headlight colour for many years. There was a claim that 'white' lights would dazzle oncoming traffic. When I was first driving in France, one had to stick yellow filters on headlights in addition to the left/ right dipping gizmo.
 

Offline turnipsock

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Why are fog lights yellow?
« Reply #7 on: 19/02/2008 11:39:39 »
Yellow filters are used in photography to increase contrast. Also, yellow goggles are used in skiing a lot to allow skiers to see the bumps a little better.

There are quite a lot of blinding car lights on the road these days. Badly adjusted lights and people not understanding the little dial thing to adjust the hight are one. People driving with fog lights on on hilly roads, fog lights light the road at low level and blind people as they come over hills.

I good trick I learnt from cycling at night, is to wear a hat with a skip. When the idiot coming the other way waits to long to dip his lights, you can tilt your head down and block out the light.

Why do BMW drive around with their fog lights on all the time? When it gets foggy, they turn on their dipped lights as well, what is that all about?

How much damage does having your lights on do to your fuel consumption?
 

Offline MayoFlyFarmer

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Why are fog lights yellow?
« Reply #8 on: 19/02/2008 15:35:21 »
some cars you can't actually turn the fog lights off.  however, they are so low, i can't say I've ever been bothred by anyone's.

as far as fuel consumption, it technically DOES decrease you gas milage to have electrical apliances on while you are driving, but unless you have a VERY inefficient alternator, it should be rather negligable.
 

Offline JimBob

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Why are fog lights yellow?
« Reply #9 on: 19/02/2008 18:35:49 »
I do hate the halogen lights. Even on low they are blinding. I would prefer the yellow to the blinding white.
 

Offline Bored chemist

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Why are fog lights yellow?
« Reply #10 on: 19/02/2008 19:12:12 »
If this
"Water absorbs blue/green light"
 were true, then water would be red/ orange.
 

Offline turnipsock

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Why are fog lights yellow?
« Reply #11 on: 19/02/2008 22:08:23 »
some cars you can't actually turn the fog lights off.  however, they are so low, i can't say I've ever been bothred by anyone's.

as far as fuel consumption, it technically DOES decrease you gas milage to have electrical apliances on while you are driving, but unless you have a VERY inefficient alternator, it should be rather negligable.

I usually get blinded by them when they come over the brow of a hill.

I'm guessing the lights on a car consume about 200W and an alternator would be around 75% efficeint. Anyway it is just a pointless waste of fuel. Usually, when a car has a slack belt, the load of the lights and the extra power required to recharge the battery from the effort of starting are enough to make the belts slip and screach.
 

Offline DoctorBeaver

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Why are fog lights yellow?
« Reply #12 on: 19/02/2008 22:17:34 »
Why do BMW drive around with their fog lights on all the time? When it gets foggy, they turn on their dipped lights as well, what is that all about?

Speaking as a BMW driver, I have never yet used my front fog lights. I have, though, used the rear one during a particularly heavy snow fall to make my vehicle more visible from behind.
 

Offline JimBob

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Why are fog lights yellow?
« Reply #13 on: 20/02/2008 00:09:30 »
Why do BMW drive around with their fog lights on all the time? When it gets foggy, they turn on their dipped lights as well, what is that all about?

Speaking as a BMW driver, I have never yet used my front fog lights. I have, though, used the rear one during a particularly heavy snow fall to make my vehicle more visible from behind.

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Why are fog lights yellow?
« Reply #14 on: 20/02/2008 08:40:38 »
Why do BMW drive around with their fog lights on all the time? When it gets foggy, they turn on their dipped lights as well, what is that all about?

Speaking as a BMW driver, I have never yet used my front fog lights. I have, though, used the rear one during a particularly heavy snow fall to make my vehicle more visible from behind.

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Offline chris

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Why are fog lights yellow?
« Reply #15 on: 25/02/2008 01:34:36 »
Another Someone has made a slip - water does NOT absorb blue light - otherwise, as Bored Chemist points out, the sea would look RED!

Instead I think Sophie Centaur is right and it's all to do with scattering. The effect was discovered by the Irish scientist John Tyndall and now bears his name. What he found is that shorter wavelengths of light (i.e. towards the blue) are scattered more by small particles than longer wavelengths of light (i.e. red).

So if lights are white then the shorter wavelength fractions (towards the blue) in the light will be significantly scattered by the fog (water particles suspended in the air). This is why the lights of a car approaching you in dense fog tend to look more red from far away (because white light minus some blue is redder). But for the driver of that car, however, the glare of the fog directly in front of the car looks blue because the fog is scattering the blue light from the headlamps.

But using yellow lamps would, in theory, reduce the proportion of wavelenths that are scattered, cutting down the glare for the driver.

I'm not sure how reliable this is however - take a look at our Why Streetlights make things look Orange experiment:

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/why-sodium-street-lights-make-things-look-orange/

Chris
« Last Edit: 25/02/2008 01:39:18 by chris »
 

Offline lightarrow

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Why are fog lights yellow?
« Reply #16 on: 25/02/2008 12:56:28 »
Another Someone has made a slip - water does NOT absorb blue light - otherwise, as Bored Chemist points out, the sea would look RED!

Instead I think Sophie Centaur is right and it's all to do with scattering. The effect was discovered by the Irish scientist John Tyndall and now bears his name. What he found is that shorter wavelengths of light (i.e. towards the blue) are scattered more by small particles than longer wavelengths of light (i.e. red).

So if lights are white then the shorter wavelength fractions (towards the blue) in the light will be significantly scattered by the fog (water particles suspended in the air). This is why the lights of a car approaching you in dense fog tend to look more red from far away (because white light minus some blue is redder). But for the driver of that car, however, the glare of the fog directly in front of the car looks blue because the fog is scattering the blue light from the headlamps.

But using yellow lamps would, in theory, reduce the proportion of wavelenths that are scattered, cutting down the glare for the driver.

I'm not sure how reliable this is however - take a look at our Why Streetlights make things look Orange experiment:

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/why-sodium-street-lights-make-things-look-orange/

Chris

It could be, but it's difficult for me to think of it as a strong effect, I have always seen the glare of the fog as white, with white lamps on. Instead, if we could use red lamps I think there really be an effect of greater light "penetration", as others wrote (but we can't, it's forbidden).
Maybe the main reason is the greater eye's sensitivity to yellow colour, as George suspected. However, that is true for daylight vision. With low light (e.g. night vision), the eye is more sensitive to green-blue (Purkinje effect) because rods (which "come into action" at dusk) are more sensitive in that range.
« Last Edit: 25/02/2008 13:05:25 by lightarrow »
 

Offline Bored chemist

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Why are fog lights yellow?
« Reply #17 on: 25/02/2008 19:31:32 »
My pet theory is that it distinguishes them from headlights and tail/brake lights.

The effect ofwavelength on scattering by particles comarable in size with or bigger than the wavelength of light is small.

The human eye is more sensitive to yellow light than to other colours, but you don't gain sensitivity (for a given power of lamp) by filtering out the red and or blue light.

Streetlights are yellow because there happens to be a really efficient way of making yellow light easily.
 

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Why are fog lights yellow?
« Reply #17 on: 25/02/2008 19:31:32 »

 

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