79
$\begingroup$

Here is a picture of my power adapter. You can see in has one green LED lit when charging.

enter image description here

Now here is a picture of my mirror with beveled edges. When I view the power adapter in the mirror, I see three (3) projections of the LED: the original, plus two "ghost" LEDs (one on either side).

enter image description here

Can someone explain how the mirror's bevels are able to cause these triplicate projections? And why does it only appear to affect the LED (except near the edges)? If the mirror had a different bevel pattern (for example, in the following image), would it affect the number of "ghost" projections?

enter image description here

UPDATE: For what it's worth, I took another picture late at night, with the camera lens as close to the mirror as possible to maximize the angle of reflection. Doing this showed me 6 (!) ghost projections, which is (mostly) in line with @Agent_L's answer (There are actually many more "after", but as every next one is, let's say, 90% dimmer than the previous, only the first is noticeable.). Apologies for the poor image quality.

enter image description here

Improve this question
$\endgroup$
3
  • 4
    $\begingroup$ I like the way "Ogee" has been initialized as "OG" in the product catalog. $\endgroup$
    – JPhi1618
    Commented Sep 12, 2017 at 15:13
  • $\begingroup$ I think I see a duplicate on the shadow of the cable, too. $\endgroup$
    – Stack Exchange Supports Israel
    Commented Sep 15, 2017 at 12:03
  • $\begingroup$ Thanks for the second image! The first few appear as having same maximum intensity, because they all exceeded brightness range of your camera. It's simply "as green as it can register" and everything over the top got ignored. Eye can't do that, because it'll more or less "recalibrate" itself to the brightest spot. 6 is the total number, but only 4 of them are "afterghosts". Also, if your mirror is made from high refractive index glass, the "parasitic" reflections will be much stronger. It could be made of lead/crystal glass, to make the bevels shinier/prettier. Another coincidental relation! $\endgroup$
    – Agent_L
    Commented Sep 20, 2017 at 16:29

3 Answers 3

Reset to default
144
$\begingroup$

I believe this is what's happening:

drawing

  1. The first "beforeghost" is the faint reflection off the surface of the glass.
  2. The second solid image is the intended reflection off metalized layer.
  3. The third "afterghost" is the faint internal reflection off the inside of the glass, then properly reflected again off the metalized layer.

There are actually many more "after", but as every next one is around 96% dimmer than the previous (exact number courtesy of Jan Hudec), only the first is noticeable. They're all spaced nicely and evenly, because this spacing is determined by the thickness of the glass.

This effect has only coincidental relation to the beveled edge. If the glass is very thin, all images appear so close to each other that they appear as blur, not distinct images. The glass has to be quite thick - and thick mirrors tend to have large, decorative bevels. (Also, the observed object needs to be small and standing out - just what your LED is.)

Similar effects happen in eyeglasses, and those are sometimes specially treated to minimize it, known as "antireflection coating".

An effect similar to "afterghost" is deliberately exploited in so called "infinity mirrors". Instead of internal reflection they employ a second, partially transmissible mirror faced inside to increase the reflection into useful levels.

Improve this answer
$\endgroup$
3
  • 1
    $\begingroup$ OP: Try turning off the light: Do you see the 3rd ghost? Change the angle, so the light passes through more or less glass, and see if the distance between the ghosts change, too. $\endgroup$
    – Ole Tange
    Commented Sep 11, 2017 at 10:58
  • 3
    $\begingroup$ Note that this is similar to how Rearview Mirrors work when they are altered for bright lights. $\endgroup$
    – David Starkey
    Commented Sep 11, 2017 at 19:27
  • 2
    $\begingroup$ IIRC reflection from glass surface is around 4%. So the ghost before and the first after are around 4% of the main reflection and then each is 96% dimmer. $\endgroup$
    – Jan Hudec
    Commented Sep 14, 2017 at 11:13
42
$\begingroup$

Yes, good eye!

My suspicion is that the ghost images are from reflections off the glass on your mirror. The entire image is getting ghosted (not just your LED), but the ghosts are weak (~5% of the normal reflection), so you can only see it for the bright green light on the black background.

So why is there a ghost to the left and right of the image? Remember, your mirror consists of a clear slab of glass with a smooth metal film on the back. The "regular" image you see comes from the light that goes through the top glass interface (air-to-glass), reflects off the metal, goes back through the glass interface (glass-to-air), and gets to your eye. The ghosts come from extraneous reflections at those glass/air interfaces (5%, as I mentioned before). The first ghost comes from that first interface (appearing to one side of the regular image). The second ghost comes from the second interface, sent back to the metal (appearing to the other side of the regular image). There will actually be more reflections, 3rd and 4th etc., getting progressively (and quickly) weaker.

Check out Floris' answer to this question.

I don't think the bevel is relevant for the ghosts around your LED. I'll say it again: Good eye! And good for you for wondering about it.

Improve this answer
$\endgroup$
20
$\begingroup$

This is kind of hard to answer from only this one picture, but I'll give it a shot.

My guess is that the centre LED in the mirror is the one reflected by the actual reflective coating (most likely silver) at the back of the mirror. The one appearing to be closer to the mirror is the reflection from the bare glass surface that you would see even without the coating. Just like you see yourself in your windows when it's dark outside.

The one appearing farther from the mirror (i.e. the rightmost in the picture) is then probably the same effect but for the light coming back from the silver through the glass, reflected at the glass-air surface, that is then again reflected back to you from the silver coating.

The reflectance of an air-glass surface is very low (only a few percent), so the light source must be really bright in order to see this effect. Therefore it only appears for the LED, which is probably much brighter than any other objects in the room.

As you might have already guessed, this has nothing to do with the way the edges of the mirror are cut.

Improve this answer
$\endgroup$

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