Georg, could you please clarify some of your points?

You say: ‘Sodium lamps ... work at a rather high pressure, in order to have as much "pressure line broadening” as possible. ‘ At first I was puzzled at your apparently not knowing that there were different kinds of sodium lamps, for example LPS and HPS, which certainly do not both strive after pressure line broadening. Then you later referred to LPS in a different context, but not the context under discussion.

I cannot remember anyone asking about German lamps in particular, and you do not explain why you mentioned them in particular. You also said that ‘In Germany at least there is no "former use" of sodium lamps on streets, conrarily, they are used more and more.” No doubt. I have not been in Germany lately and am happy to take your word for that. No doubt as newer technologies come to the fore they will be used less and less.

However, the lamps under discussion were the vividly yellow LPS lamps, and you then say, surprisingly in the light of your foregoing statements: “The classical low pressure sodium lamps are phased out of use”. It is not clear in context why you denied the “former use”. Please explain. In particular please explain what it was that you seemed to intend to contradict, or if not, what you intended to say at all.

For my part, I do not think that the phasing out of the LPS is such a good idea during the period in question anyway. They were cheap, effective, long-lived, and very efficient in comparison to HPS lamps. They don’t seem to interfere with astronomy as badly as their rivals. They were not especially pretty, but I suspect that even today intelligent planning could make good use of a mixed system, with LPS in combination with other lamp designs. Granted, “intelligent planning” is a big ask... (On balance, for example because of improved lamp life, more recent advances, such as fully practical induction lamps or possibly LEDs might be better, but they were not available at the time I mentioned. In the long term there always is improvement, though the long term sometimes is very long.)

As long as the bulb is intact, the yellow is dominated by the other wavelengths from the fluorescent layer, but I would have expected the entry of air to quench the sodium yellow too fast for you to see.'

Sodium lamps never have some fluorescent lining.

They work at a rather high pressure, in order to have as much

"pressure line broadening" as possible.

In Germany at least there is no "former use" of sodium lamps on streets,

conrarily, they are used more and more. The classical low pressure

sodium lamps are phased out of use.

Georg

I don't think this has anything to do with the lamp itself. I think that what you are seeing is an after image created when the light is suddenly put out. The colour of the after image will depend upon the color content of the light from the lamp which will depend on type of coating on the inside of the tube. This is much the sam effect that you get if you shine any bright light in your eye and then suddenly turn the light out. The after image will probably last for about 1/25th of a second and will look like a flash to you.

I find your observation surprising, partly because I am not much in the habit of smashing fluorescent bulbs while they are switched on. However, the way they work actually is largely by fluorescence, hence the name. The way this works is that the gas that is excited by the electric current largely produces ultraviolet, which, also largely, is not visible to us, apart from being harmful if it could reach us. In any event, hardly any of the light produced is the light we see; instead what happens is that the ultraviolet excites the molecules of the substance lining the glass of the bulb, making it give off a mix of light frequencies that we find useful and harmless.

Some classes of fluorescent bulbs use sodium vapor as a component of their electrically excited gas. Such a yellow used to be popular for street lighting. It certainly is true that the glowing sodium vapour then shines yellow, though I am surprised that you should see that as a flash on breaking the bulb. As long as the bulb is intact, the yellow is dominated by the other wavelengths from the fluorescent layer, but I would have expected the entry of air to quench the sodium yellow too fast for you to see.'

Well, if you are right, then congratulations on your observation, and thanks for sharing.

Go well,

Jon

Your point has merit of course, but it is not really cogent. The semantics of the word "storage" are largely arbitrary, but I suggest that the most nearly essential attribute is that one begins with some entity and eventually recovers that entity.

Consider a tent for example: among many possible definitions we could call it a dwelling, in particular, a portable dwelling. So let's port it! First, we knock it down, fold it up, and in doing so reduce it to anything but a dwelling except possibly for a few enclosed cockroaches. When we reach our destination we unfold it and re-erect it.

Now, would you deny that we had stored the tent, on the grounds that during its storage it had not been a dwelling? I think that is rather special pleading.

In a delay line storage device a modulated beam of light is ideally not changed at all, either in frequency or in modulation. The fact that it is moving does not seem to me a persuasive argument for its not being stored. Certainly the data, the information, is stored, unless you have some argument to the effect that moving data is not stored data. Suit yourself on that point, but it is just possible that you might be in the minority in insisting on any such convention.

It certainly is true that stationary light is something of a contradiction in terms, much as warm ice would be. Where I come from, ice, on being warmed, turns into liquid and putting it into a glass to prepare say, whiskey on the rocks, would not satisfy most clients. Accordingly, although I could store the water content of ice, if I did not store the ice cold, I could hardly claim to have ice in store.

Similarly, although I could store either the data from a modulated beam of light, or its energy, or even both, permitting me to regenerate a reasonably equivalent facsimile, to claim that I was storing the light itself would be a rather peculiar argument if the light were not moving, any more than the ice would be ice if it were molten.

In short, I don't see why storing something in a mode that maintains appropriate aspects of its nature should not be called storage simply because it is not static.

But by all means, if this does not strike you as reasonable, please do not feel constrained to adopt my personal preferences.

Cheers,

Jon

Ah, but what does it mean to "store"?  Presumably to keep in one location in space for an indefinite time. Photons, in their raw form, cannot exist without movement (from our perspective). A photon has no concept of time: while it may seen to us that it has traveled from the middle of the universe to us in 13B years, to the photon, it got there the instant it was created.  From the photon's perspective, that vast distance is exactly zero meters, so of course it took zero time.  From the photon's perspective, it was very well stored.

There have been successes in slowing light down, stopping it, and even getting it to move backwards, but this is traveling in a medium while the medium moves.  (look up Dr. Lena Hau) I would not call this light in it's raw form

--Dave

While I agree with the foregoing replies, I have a reservation. You can store light. It actually gets done in ceertain data storage devices, though I am not aware that any of them have been used commercially. They are versions of the technological  strategy  known as delay line storage. The trick is to send information-bearing beams of light down a suitable channel, typically by  repeated reflection, then to capture them before they fade, and re-project them as long as needed. This stores the data for as long as required, though no one beam of light lasts longer than a small fraction of a millisecond.

But then you did ask about storage, not indefinite storage. For longer-term storage, consider optic fibres. It is now possible to store a beam in a fibre for several milliseconds. For light that is not bad going at all. It would be ungracious to complain about its shelf life. You don't say that we cannot store a drinkable cup of coffee, just because it goes off its best in less than an hour, do you?

If we can  manage a suitably high refractive index in a clear medium, we could store light for much longer. Bob Shaw wrote some science fiction on that theme. He did a pretty creative job of it too.

The other theory, of light travelling in electromagnetic waves, presents a problem. The light's amplitude fades over distance and even though they can travel for immense distanced in a vacuum, they can not be completely kept. However, if you are talking about in a human lifetime, then they wouldn't lose their amplitude then, or even for millions of years. This is why we can still see stars, even galaxies on the other side of the universe today.

If you were looking to store the light in a mirror box solution, then you would have to diffract the light around, which wouldn't be easy as the gap would have to be shorter than the wavelength of the light itself. As the beams aren't naturally polarised, only a select few waves would diffract the first few hundred times, with less and less over time. A mirror could be used, but only 99% of the light is reflected back.

Due to this problem, it would be extremely difficult to store the raw light.

Harry