2
$\begingroup$

I recently came across this image: https://i.sstatic.net/fNGS4.jpg and I am having trouble understanding why this would work.

My understanding of the combination of a green and red color filter is: the green filter absorbs anything but green light, the red filter absorbs anything but red light so no light should be visible behind this combination of filters.

Obviously for this turn signal this is not the case, but why?

Improve this question
$\endgroup$
3
  • $\begingroup$ I think you are mixing a couple of things. First of all the colours that you (or in the case of the image; the camera) sees is basically the reflected sunlight. Therefore the "green" plastic reflects (predominantly) green light and "red" plastic predominantly red light. Absorption as you write can only happen, if light transmits through both materials, so if the lights of the car were turned on. $\endgroup$
    – franz
    Commented Nov 25, 2021 at 10:35
  • $\begingroup$ Concerning your question; I don't think that these plastics are real colour-filters. They are basically plastics with pigments added. These pigments interact with light and enhance certain wavelengths (e.g. colours of light) but the background radiation is still a diffuse mixture of other visible wavelengths. Therefore there is still 'enough' red (and basically all other colours as well) left after the green plastic. $\endgroup$
    – franz
    Commented Nov 25, 2021 at 10:36
  • $\begingroup$ I am talking about the case that the light of the turn signal turns on. In that case the light passes through 1. the green plastic, 2. the red plastic $\endgroup$
    – joekr
    Commented Nov 25, 2021 at 10:40

1 Answer 1

Reset to default
3
$\begingroup$

You are right that good red and green filters, like used in fluorescence spectroscopy, should let no ligth pass through.

But coloured plastic usually does not make a perfect narrowband filter: there is probably some spectral overlap between the pass-band of the green plastic and of the red plastic. As a result, a portion of yellow light will come out as turn signal.

Most of the light gets absorbed in either filter. It is energetically wasteful, but cool design features like this tend not to be technically optimum in general.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.