Sodium vapor lamps - how do they work

  • #1
Flying Penguin
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I don't quite understand the explanation at wiki;
when the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright orange/yellow color as the sodium metal vaporizes.

What does this mean (the vaporizes bit)? the sodium turns to gas, which then glows? when it cools does it condense back to a solid when its turned off again?
 

Answers and Replies

  • #2
ZapperZ
Staff Emeritus
Science Advisor
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Flying Penguin said:
I don't quite understand the explanation at wiki;

What does this mean (the vaporizes bit)? the sodium turns to gas, which then glows? when it cools does it condense back to a solid when its turned off again?

First of all, can we please make sure people read Wiki with CAUTION? While there are people with good intentions of making sure the entries there are accurate, such quality control isn't guaranteed! There's no substitute for good old standard physics text.

Now, to get back to the question. Discharge tubes usually contain the vapor element already, or at least, have the ability to heat up a source to produce the vapor. Once we have that vapor, a number of things will occur. You have a cathode element that, upon heating, will produce electrons. At the other end of the discharge tube will be an anode that these electrons will be accelerated and attracted to. Along the way, these electrons will collide with the vapor in the tube. This collisions will impart energy to the vapor, causing an atomic transition to an excited state. Since such states are unstable, the atoms will decay back to the ground state. That process will emit light, and will emit visible light if it happens to fall within that range.

Such process will continue as long as a potential difference is applied to accelerate the electrons.

Zz.
 
  • #3
Antiphon
1,683
3
Flying Penguin said:
I don't quite understand the explanation at wiki;

What does this mean (the vaporizes bit)? the sodium turns to gas, which then glows? when it cools does it condense back to a solid when its turned off again?

At first the arc tube is cool and there is very little vaorized sodium. Most
of it is in a pool condensed on the sides of the arc tube. The light emitted
is a functiuon of the current and the sodium vapor pressure. As the tube
heats up, the rest of the sodium vaporizes and the efficiency of the tube
(at converting electric current into light) goes up dramatically.

And yes, it does condense and mostly solidify again when it's turned off.

Even more interesting are Metal Halide lamps which contain things like
sodium-scandium and iodine, mercury and other compunds. These lamps
give off nearly pure white light and are much better to work under than
sodium lamps. The extra light spectrum comes from the fact that there
are molecular vibrations as well as electronic transitions in these more
advanced lamps.
 

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