Years ago my wife and I rode unambiguously
vivid tomato red motorcycles. It happened to be the available colour. On a
journey one night, we got onto a road illuminated by sodium discharge lamps
that were popular in those days because they economically produced an intense
yellow light. Nowadays street lights usually produce apparently white light; I
don't know when I last saw a string of yellow lights at night. Mind you, most
of the street lamp technologies currently in use do not produce true white, but
a very spiky spectrum indeed. As far as our eyes can tell, it is white, but the
effect on the appearance of something that reflects mainly colours that lie
between the spikes, can be to make it look either dark or some unnatural
colour. Beware of swearing to colours seen under streetlights or being too
confident of correctly guessing at their colour as seen in sunlight.
Anyway, the effect on our driving
vision was not obvious; certainly the outlook tended rather towards yellow, but
that was not disturbing. In fact I would never have given it a thought, except
that we soon noticed that the unambiguous tomato red of our mounts had given
way to an equally unambiguous dove grey. The effect was so marked, that we
stopped to inspect. There was no way to persuade our eyes that we were looking
at anything remotely red. After a while we noticed that just a scrap of red showed
on the mudguard where the headlight caught it. Otherwise the only way to "see
red" was to turn our headlights on each other's vehicles.
By far the bulk of the light emitted
from the lamps was sodium yellow with a hint of yellow green and a bit of whitish
noise in various other colours. In contrast to the vivid sodium yellow, that
little fraction of the light that reflected from our bikes, that otherwise no
doubt would have looked simply grey, presumably gained a bias in favour of the
colour opposite of yellow, giving it a bluish tone, hence the dove grey, rather
than dead grey.
A related laboratory experience when
I was working on the distribution of aerial sprays, involved looking through a
microscope at coloured marks on white backgrounds. The pigments in use were
chosen for their different solubilities and their contrasting colours. I chose red
and green, which worked very well. Seen through a red filter, red marks simply
vanished on the white background, while green marks looked blackly obtrusive. Looking
through a green filter gave the opposite effect.
Very well.
Seen in daylight or under an
ordinary incandescent light, your violet amethyst might very well look violet.
You can create a very similar colour in ordinary glass by melting small impurities
of manganese and possibly iron into it. You also might experiment by showing
your amethyst to friends with various forms of red green colour blindness. I
suspect that at least some of them would say that your amethyst is blue. I know
someone who is red-green colour blind and sees red port wine jelly as blue. Only after he said so did I notice that in fact
the jelly's red did have a blue tint. We could see your amethyst problem in
a similar light. Daylight supplies all visible frequencies and the amethysts mainly
pass blue and something more reddish, giving violet. Your laboratory fluorescent light
probably supplies lots of blue and very little of the correct shades of red
light. Your other "kind of light"
probably had one of those spiky spectra that favoured both colours but missed a
lot of others. As amethyst is at most very weakly fluorescent, fluorescence is
unlikely to have any serious influence. Amethyst being essentially slightly impure
crystalline quartz, it does however have noticeable effects on the polarity of light, rotating the
plane of polarisation and so on, but since the light sources you mention were
not polarised, that is unlikely to have had much effect.
Enough for now. If you have any more
information, let us know.
Go well,
Jon
