Salt lamps - what effect would the salt have on the light from incandescent bulb

[Voltaic]

A salt lamp is a solid chunk of salt (with some minerals in it) with a hole drilled in the middle where an incandescent bulbs goes.
So the light shines through quite a few cms of salt before leaving.
The salt lamp appears to be a lower colour than the bulb would be by itself.

What is actually happening, is the light reduced in frequency (nm) by going through the salt? Or is that impossible?
Is it just filtering out certain wavelengths, if so which ones would be filtered?

 

[Harald Kapp]

What is actually happening, is the light reduced in frequency (nm) by going through the salt? Or is that impossible?

That's not impossible - frequency changing materials exist - but veeeery unlikely.

Is it just filtering out certain wavelengths, if so which ones would be filtered?

This is the main effect responsible for the shift in color. Which wavelengths are filtered depends on the composition of the salt/mineral block. A simple do-it-yourself test uses a prism to generate a rainbow spectrum. Compare the spectrum from the unfiltered bulb wit the spectrum from the lamp to find the difference.

 

[Voltaic]

What is the scientific term for the light wavelength(s) that a material allows to pass through it or blocks?

I'll look at getting a prism just for interest, I hope it works with such an omni directional source.

What wavelengths would you expect pure salt to block/pass? I assume it blocks the higher ones and passes the lower ones.

 

[Harald Kapp]

What is the scientific term for the light wavelength(s) that a material allows to pass through it or blocks?

I think it is called absorption spectrum. As for the expected absorption spectrum I'm sorry that I can't help.

 

[Voltaic]

Thanks

 

[witsender]

Hi Voltaic
The visible absorption spectrum could be measured using a camera.
The light recorded in photographs of the unshaded lamp and the same lamp shining through the salt block would be compared using photoshop or gimp (a freeware photoshop-like application).
The comparison would be done using the colour correction facility.

I'd be pleased to assist - if you post the photographs I'll post my results (and my exact method).

 

[hevans1944]

Salt is also quite transparent to infrared to about 16 μm wavelength, so a secondary effect is you should be able to feel this as a warming effect on your hands through the salt from infrared radiation emitted from the incandescent bulb. We used to use precision-ground salt lenses for infrared work, but they needed careful handling and of course were easily damaged by exposure to moisture. Thankfully, zinc selenide was soon available in massive quantities so salt lenses pretty much went away for use in the 1 μm to 10 μm wavelength ranges, except for those who couldn't afford ZnSe, which is roughly ten times more expensive.

 

[KJ6EAD]

Pure salt will tend to yellow the light a bit but since your incandescent source is rich in those wavelengths without filtration it may not be noticeable. The typical salt specimens used for lights have some iron compounds in them which will naturally filter to orange and rusty red.

 

[Voltaic]

Thanks, what happens to the light the iron absorbs, does it get re-radiated as a small amount of far infrared?

 

[KJ6EAD]

There is no elemental iron, only compounds thereof. These compounds filter out the small portion of the light from the incandescent source that are higher frequency wavelengths such as purple, blue and green, converting them to thermal energy mostly in the longwave IR spectrum.