Cathode Ray Tube: Explaining Phenomena with Green Beam and Fluorescent Glow

In summary: The green probably comes from the phosphor.In summary, cathode rays appear as a green beam in a video demonstration and can be deflected by a magnet. The electrons that make up the beam excite gas atoms in the tube, causing them to glow. The glowing is more visible with less gas pressure. In the paddle wheel demo, the cathode rays hit the wheel and cause it to rotate in both forward and reverse directions. When a maltese cross is placed in the path of the cathode rays, it creates a dark shadow on the fluorescent screen, but when flipped down, the previously shadowed area glows brighter due to the absence of the shadow. The tube in the demo has a fluorescent stripe inside, which the electrons
  • #1
ananthu
106
1
In a video demonstration (not animation), cathode rays appear as a beam of green light moving from the cathode to anode inside the tube. The demonstrator brings a magnet near the tube and the green beam deflects. This I can understand. What I can not understand is that how the electron beam which are invisible are made to appear as a green beam and the beam also beautifully bends. What we have read is the fluorescent glow appears only when it strikes the glass. But how the path of the cathode rays in the gaseous part of the tube glows?

2.In the paddle wheel demo. when the cathode rays hit the wheel it rotates and moves in the forward direction, but how it moves in the reverse direction?

3.When a maltese cross is placed in the path of the cathode rays, a dark shadow in the shape of the cross appears on the fluorescent screen.When the cross is folded down, a fluorescent glow in the shape of the cross appears in the same place previously occupied by the shadow. I could understand the previous phenomenon but not the latter.

If anybody gives simple explanation for the above questions, it will be helpful to me.
 
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  • #2
But how the path of the cathode rays in the gaseous part of the tube glows?
You need a vacuum for the rays, but the vacuum is not perfect - electrons can hit the remaining gas atoms/molecules inside and excite them.

For 2./3., I think there is some context missing.
 
  • #4
ananthu said:
Thank you for your reply. Even if there is some traces of air, will it glow green, when struck by electrons?

Regarding the (2) and (3) I have given the video links below.

(2)

(3)http://www.youtube.com/watch?v=Xt7ZWEDZ_GI&feature=results_video&playnext=1&list=PL1559C68545F8F4A1


But the glowing will decrease. For the cathode rays to be constituted one needs very very less (nearly vacuum) gas pressure. Otherwise some gas atoms can even take electrons. It creates a kind of "viscous drag".

But with very less gas pressure , this viscous force is reduced to a great extent and the "thermions" which strike the gas atom can ionize them , i.e. knock the valence electron from their orbital.
 
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  • #5
For (3) the explanation is right there on youtube:

"The after-shadow is because the glass loses some of its fluorescence under bombardment by electrons. When the cross is flipped down the previously shadowed glass is exposed to the electron beam and glows brighter than the surrounding glass. With this tube there isn't any phosphor, just natural glass fluorescence"
 
  • #6
ananthu said:
But how the path of the cathode rays in the gaseous part of the tube glows?
If the tube is like the one I know, there is a fluorescent stripe inside the tube.
Some electrons hit this stripe and "trace" the path of the beam.
Does it look like this one?

http://web.chem.ucsb.edu/~feldwinn/DemoLibrary/Chapter2.html
 
  • #7
Thank you for all the replies.
 
  • #8
nasu said:
If the tube is like the one I know, there is a fluorescent stripe inside the tube.
Some electrons hit this stripe and "trace" the path of the beam.
Does it look like this one?

http://web.chem.ucsb.edu/~feldwinn/DemoLibrary/Chapter2.html

You are not actually looking at a pencil beam in that demo. The beam of electrons is in the form of a horizontal fan and the screen is suspended in the tube at a slight diagonal angle so different bits of the fan strike the screen at different distances along the tube - giving you a trace all along the screen. (Like when you shine a torch obliquely along a wall) This shows how a single pencil beam would behave when you introduce magnetic and electric fields to deflect it. It's a clever idea but can actually confuse people if it's not actually explained.

btw, the 'maltese cross' tubes that I have seen have the end face coated with a phosphor. It makes the green cross much more visible. Also, as I recollect, the colour you get from electrons passing through low pressure air is Purple / Pink and not Green.
 
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1. What is a cathode ray tube (CRT)?

A cathode ray tube is a vacuum tube that uses electrons to create an image on a screen. It was the primary technology used in television and computer monitors before being replaced by newer technologies such as LCD and LED.

2. How does a CRT produce a green beam and fluorescent glow?

A cathode ray tube works by using a heated cathode to emit electrons, which are then accelerated and focused into a beam by an anode. This beam is then directed towards a fluorescent screen coated with phosphors. When the electrons hit the phosphors, they cause a fluorescent glow, which can be seen as a green beam when the phosphors are coated with a green material.

3. What are some common phenomena explained by the green beam and fluorescent glow in a CRT?

The green beam and fluorescent glow in a CRT can explain several phenomena, including the formation of images on a screen, the persistence of vision effect, and the creation of color in television and computer displays.

4. How does the green beam and fluorescent glow in a CRT create images?

The green beam in a CRT can be manipulated by electromagnets to scan across the screen in a series of lines, creating a raster pattern. As the beam moves across the screen, it activates the phosphors, creating a bright spot of light at each point. By controlling the intensity and timing of the beam, complex images can be formed on the screen.

5. Are there any drawbacks to using a CRT?

While CRT technology was groundbreaking in its time, it does have some drawbacks. CRTs are bulky and heavy, making them inconvenient for modern devices. They also consume a lot of energy and can produce harmful radiation if not properly shielded. Additionally, CRTs have a limited lifespan and can suffer from issues such as image burn-in and flickering.

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