Vacuum Tubes and Light

I think it's more likely EMI from the flashlight (perhaps there's a switching DC/DC converter that steps up battery voltage to drive the LEDs, or some sort of pulse-width modulation to control duty cycle of the LEDs... I'm not really familiar with the design of LED lights) that's causing the response... I think it's very unlikely the light itself has anything to do with it.

It wouldn't surprise me for that to happen.  Vacuum tubes aren't supposed to create light, but they do because it's impossible to have a pure vacuum with 0 atoms of anything in there, so that same matter could react from photons hitting them.  Try just a regular flash light with a bulb, or an LED connected right to a battery.

Quote:

Originally Posted by ramicio 

It wouldn't surprise me for that to happen.  Vacuum tubes aren't supposed to create light, but they do because it's impossible to have a pure vacuum with 0 atoms of anything in there, so that same matter could react from photons hitting them.  Try just a regular flash light with a bulb, or an LED connected right to a battery.

Light doesn't travel in a vacuum?

Light bulbs don't work if the vacuum is perfect?

The visible (orange) light you see emitted by tubes is coming from the cathode heater (although there can be faint emissions caused by fluorescence or ionization.) But emitting light and being affected by it are two different things and I'd be surprised if shining a light on a tube would have much effect. I agree with tlniec, I think there's something else going on, perhaps some electromagnetic effect.

You misunderstood what I meant.  I mean if vacuum tubes had a perfect vacuum with no matter whatsoever in them, then light would no be created by them.  The light created is interactions with electrons and whatever matter is present in the impure vacuum of the tube.

I don't think that's 100% true.  As ilikemusic said, the cathode heater would still emit light even in a perfect vacuum.  Similar to the filament in a tungsten incandescent light bulb, you have a situtation where electrons in the metal atoms are excited/energized, and when they drop back to their 'ground state' they happen to give up some of their energy in a frequency that falls in the visible light spectrum.

Some of the light in some tubes, usually diffuse glowing in colors other than red/orange/yellow, is caused by impurities in the tube that remain after the vacuum sealing process. But mostly, the glow from tubes comes from a thoriated tungsten cathode, oxide coated tungsten cathode, or tungsten heating element with enough current running through it to run temps in the 1000 to 2000 C range. Things in that temp range will glow anywhere from a dull orange-red to a bright yellow-orange.

"Hey guys, I'm adding energy to a system. Why does it change?"

There's some sort of EMF created by your flashlight, it's not bright enough to significantly effect your system.

Hot cathodes do still glow in vacuum, the metal evaporator I occasionally use has a very high vacuum and you can see the glow.

Vaccum tubes are not normally sensitive to visible or invisible (UV or IR) light. Solid State transistors on the other hand are often sensitive to a very wide spectrum of light, ocasionaly with destructive results. This is why transistors are ALWAYS entombed in opaque plastic, and you occasional see LED's which are painted black.
 

Quote:

Originally Posted by MrGreen 

"Hey guys, I'm adding energy to a system. Why does it change?"

Sadly it did not inhibit you from logging into head-fi and spewing worthless drivel.

Any real scientist would have found my post hilarious. Say that at a gathering of physicists and they'll all laugh.

Photomultiplier vacuum tubes are absolutely great at detecting extrernal light. Some of

them have photon gains in the billions.

But even regular vacuum tubes can detect photons.  In fact many moons ago, tubes

used as the first stages in high gain amplifiers were painted with a thick heavy

black paint to prevent this.

Setup a 6dj8 circuit, voltage gain of 100.  Look at the plate voltage.

Hit the plate with a laser, even a pocket pointer works. But the

higher power ones work even better.

light definitely travels very well in a vacuum. Much better than it does in air in fact.