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I will try and describe my observations the best I can.

I placed a candle in front of me and my spectacles a bit further ahead of the candle. I have negative power so my spectacles must be concave. I tried to see the image formed from behind the candle and noticed two things :

  1. There were two images formed, one of them was noticeably further away than the other.
  2. the image that was further away was green in colour while the original flame was yellow (I believe the technical term is blueshift?)

For the first observation, I came across the concept of a "virtual object" (or as an answer here suggests calling it the "would-have-been image") defined for a convex lens. Using the same logic for a concave lens, I can see why there will two images formed at some distance away from each other. However, I cannot think of any reason for that change in the colour of the second image (A friend suggested Doppler effect but I am thoroughly unconvinced).

Could someone please explain the second observation and correct me if I am wrong about the first one?

A picture of my experiment

enter image description here

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  • $\begingroup$ Doppler effect is not due to lenses but is a change in color (frequency) due to the shift in reference frames, and requires more sensitive apparatus to observe then the human eye. In case of lenses, [en.wikipedia.org/wiki/Chromatic_aberration] (Chromatic aberration) might be the cause of this phenomenon. $\endgroup$ Jul 29, 2020 at 11:38

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The two images correspond to reflections from the two different surfaces.

One image has essentially unaltered color. That image is due to reflection from the lens surface closest to the candle.

The other image has an altered color, and is due to internal reflection from the back surface of the lens. Your eyeglasses have a thin coating that transmits some wavelengths better than other wavelengths, resulting in the colored second reflection which has passed through the coating. The first reflection does not pass through the coating.

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