To reduce the intensity of the incident light, prismatically shaped rear view mirrors are used in cars with a day-night switch which changes the angle such that instead of the polished surface reflecting, it is the glass surface reflecting, as explained in the picture. But my experience states that using the switch when the light from the shiny surface is incident, always changes the configuration so as to allow the non-glary light be incident, even if the switch was in night mode to begin with.

IN other words, day -> night and night -> day switch both brings about the same change in what is observed if we begin with the shiny surface reflecting. I am unable to see this according to the diagram given, where only day -> night switch can do that.

According to the images, one position of the switch is such which that the shiny part's reflection is falling on to the eyes. On flipping the switch the mirror shifts slightly and then, the dim reflection falls on the eyes. Lets call the former position of the switch, day mode and the latter night mode. If the mirror is adjusted to show the shiny part in day mode (as is usually the case), flipping the switch will dim the image. But if the switch is on night mode, and we adjust it so that the shiny part creates an image, even then, flipping the switch will dim the reflection. Since in this case, the flipping is in a direction opposite to the former case, how come it still manages to dim the reflection?

enter image description here

  • $\begingroup$ I don't understand the question. You seem to have an incorrect assumption. The switch just slightly tilts the mirror and has two positions: one let's you see the dim reflection (and the bright reflection hits the ceiling) and the other let's you see the bright reflection (and the dim reflection hits your chest). The diagrams are correct. $\endgroup$
    – Digiproc
    Dec 30, 2015 at 12:02
  • $\begingroup$ @Digiproc I rephrased my question. $\endgroup$ Dec 30, 2015 at 12:30
  • $\begingroup$ Hmm, I can't imagine why it would do that unless its a more high-tech device and thus works differently. $\endgroup$
    – Digiproc
    Dec 30, 2015 at 15:03

3 Answers 3


Yeah, I know what you are talking about. There are two mirror positions that result in a dim image, one above the bright image and one below. My best guess is that there are two paths, kind of like I've drawn that both produce dim images.

enter image description here

The "lower" dim image is caused by photons reflecting from the air->glass transition, and the second "upper" dim image is caused by photons reflecting from the glass->air transition. In fact, this would mean there are a whole series of "upper" dim images, each getting dimer and dimmer.


If my interpretation of your question is correct (i.e. from the night mode you re-arrange the mirror manually to visualize the day mode), the problem is quite simple:

By adjusting the mirror you bring yourself to see the day-mode. Thus flipping the switch will bring again back to night mode. You will see that to pass manually from night to day-mode you can only tilt the mirror in one direction, and this direction is the same as the flipping one.

To be more clear:

Since in this case, the flipping is in a direction opposite to the former case, how come it still manages to dim the reflection?

The flipping is opposite, but before flipping, you already flipped the mirror - but manually.


Yes, I totally get your question, and I couldn't find the answer on the internet too. What stochastic13 meant was that, there are two dimmer reflections (one above and one below the bright reflection), which in the case of the diagram, there should only be one dimmer reflection (that is the one below the brighter one).

But we could observe that there are in fact two dimmer reflections in the actual rear view mirror in our cars. That was what stochastic13 meant when he said that "day -> night and night -> day switch both brings about the same change".

Let's say

  • day mode is when the switch is being lifted up, and
  • night mode is when the swithch is being pulled down as shown in the diagram.

In stochastic13's example was that, during the daytime, when the switch is at 'night mode/switch pulled down", we could adjust the angle of the mirror for normal daytime usage, with the bright image reflected onto our eyes. And when it's night time, the moment we flip the switch to 'day mode/switch lifted up', there's still a dimmer reflection of the headlights behind falling into our eyes. Meaning that switch can be used in reverse too, or in other words, interchangeably. According to the diagram , we should be seeing something else other than a dim reflection of the headlights behind if we were to use the mirror in this configuration.

We would love to hear the explanation on this if anyone knows the answer. Thank you.

  • $\begingroup$ This does not really answer the question. If you have a different question, you can ask it by clicking Ask Question. To get notified when this question gets new answers, you can follow this question. Once you have enough reputation, you can also add a bounty to draw more attention to this question. - From Review $\endgroup$
    – jng224
    Dec 14, 2021 at 7:17

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