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Why does the glass on my car's headlights have such a strange shape/texture? Are there physics-based explanations or insights for these features?

Wouldn't it be better if it was just transparent and even?

enter image description here

(car model: BMW E30 1981-1985)

enter image description here

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Relevant meta-post: Are engineering questions appropriate for this site? Seems debatable in this instance, but maybe Motor Vehicle Maintenance & Repair would be a good place to ask anyway/also. – Nick Stauner Jul 20 '14 at 22:27
@NickStauner Right, but I am asking for an insight/explanation/answer from physics point of view, I did not write that down - but if I post the question here, it should be obvious that I am not looking for math or engineering or technology or chemistry or economics point of view. – VividD Jul 20 '14 at 22:32
It should be, but people often aren't aware of the site's policies on scope limitations or alternative it's good you wrote that just now :) – Nick Stauner Jul 20 '14 at 22:35
See for details on headlamp design including the lenses. – user6972 Jul 21 '14 at 1:52
Where have you been for the last 15 years of automotive development? We have projector headlights for a while now and laser headlights are on their way into the mass market. In other words: it would be better when engineered right. – the-wabbit Aug 31 '15 at 22:27
up vote 10 down vote accepted

There are two parts to those lights:

  • the reflector, which gathers the bulb's output and creates as focused a beam as possible
  • the lens, which modifies that beam as desired.

A focused beam makes a lousy headlight. You only see a small patch way in front. The extra ridges in the pictured lenses are vertical, which means they will spread the beam horizontally, thus producing the curb-to curb beam we want without also blinding oncoming drivers.

Modern headlights moved most of the optical mechanics to the reflector. This way the designers can integrate the lens with the body. The math is a LOT more complex, but so is the design software. When those headlights were designed they used a pencil, a slide rule, and a lot of intuition.

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The type of headlight lenses shown in the images, with the rows of fringes, look like Fresnel lenses. This is essentially a lens with a series of fringes that act as prisms, each at a slightly different angle but with the same focal length. That has the effect of reflecting the non-directional light from the bulb in a particular direction.

I'd think the purpose of using a Fresnel lens is both to concentrate the beam in the direction the car is traveling more than a conventional lens of the same size could, and to direct the high and low beams.

If you tour an old lighthouse, the lenses used are also large Fresnel lenses. Especially before electric lamps were developed, these were needed to concentrate the light from, say, an oil lamp so that ships could see it at a distance. A conventional lens would have been too large and heavy.

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There are extensive legal requirements for the lighting pattern produced by headlamps (the US version runs to about 40 pages), and it's easier to produce that pattern using Fresnel lensing elements than either classical lenses or reflectors. – Mark Jul 21 '14 at 8:51

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