Shine a laser through a hollow tube kind of question

I think this is a science topic, I was fooling around with a cheap red laser and tried aiming it through a small piece of tubing. I noticed that no matter how well I held the two, the dot on striking a nearby surface would appear dimmer when going through the tube, as compared to without the tube.

The tube is a section from an antenna, about 5/32'nds inch inside-diameter & over 4 inches long. The laser is a pet store red pointer. I ran a Q-tip through the tube just to make sure there wasn't some lint in there and got the same result.

So if this is old news then please explain to me what's happening to the beam? The beam is less than half the diameter of the tube, and I'm reasonably certain it's not touching the inner wall of the tube.

Of course, I'm thinking of doing this again with better holding fixtures, and use my HeNe as that seems to have a more circular beam shape.

Would I actually lose less light if I replaced the tube with a 4-inch piece of optic fiber? Assuming I use that end piece that gets the beam into the fiber and not reflecting some of the emitted energy backwards... liberal use of technical terms there....

I don't think the beam should be attenuated if it really does at no point strike the tube. As you state the diameter of the tube is twice that of the beam, so one would not expect any difference.

Are you should the tube is straight? If it is not, the beam could easily reflect somewhere in the bend and -appear- to go straight through.

Could it appear dimmer because you're not seeing all of the scatter around the dot?

Thats also an option, depends on if the inside of the tube is reflective or not. You'd probably need a power meter to make certain what you see is an actual difference in power.

Yeah, I was thinking about scatter. So I need to verify if the dot lost light from it's outer edge.

The tube is fair straight, that is - when held up at arms length and looking through it, it appears to be very straight.

I might need a power meter when the difference becomes less, but at the moment it's a very noticeable change, by eye.

I haven't repeated this yet, but as I don't have any plan for the day, maybe I'll just do that right now.

Well, I finally got around to re-running the test and it turned out to be just a reflection issue.

First, using my HeNe gas tube as a source, I taped the laser tube down to my desktop and shimmed one end until I got a fairly level beam. I checked this by taping some blank paper to a 9-volt battery so as to have a movable target. Placing that right at the tube and then at the other end of the desktop to compare the beam height, while adjusting the shim. The desktop is fairly flat.

Then I taped a piece of paper at the far end of the desktop and penciled an X with the beam right at the intersection.

Then I used a stack of bland cd's to hold the metal tubing close to the right height, adding some tightly rolled paper to adjust the height. Now I can place the tubing in the beam path at approximately the right height to have the beam travel right through it's center.

Next, I cut two thin strips of wax paper to place on each end of the tubing so I can rotate tht until the beam is centered at both ends.

So after all that and making a series of careful adjustments I can definitely say that I have learned the practical value of a good optical workbench. It was a bit difficult to get this rig centered! And I think the handheld approach was just not good enough, I think what was happening is the beam was being reflected off of the inner wall and exiting near the center of the tube, but because the tube wall absorbed some of the light, it appeared dimmer.

So common sense wins after all, I just needed more information about what was happening.

Ah well, good experimental setup

Now bend the tube and see if you can make the beam go around a corner

Good work!

Careful set up, double checking, questioning your own results, replicating the experiment.

You are on a good path +1

Peace,
dave

Benm said:

Ah well, good experimental setup

Now bend the tube and see if you can make the beam go around a corner

Click to expand...

Just what I happened to be thinking about this very moment! But I only want to put a line generator at the end of the tube, and angle that about ten degrees off-axis.

I'm not sure what it's called, but it's the kind of generator that's a cylinder with a chisel-point at it's leading edge where the beam first strikes. Not one of those cheese grill types. I need to find someone selling those at a decent price.

daguin said:

Good work!

Careful set up, double checking, questioning your own results, replicating the experiment.

You are on a good path +1

Peace,
dave

Click to expand...

Thanks. I can see how precarious it is to just hold a laser in my hand doing stuff like this!

I was "experimenting" with a similar effect and my fish tank the other day. The glass on the tank is about 1/2 inch thick and the edges are exposed where the panes meet in the corners. Shining a laser at an angle on the edge causes the glass to act as a prism and reflect the beam back and forth inside the glass down the length of the pane. It's a pretty cool looking effect but I'm not positive the fish agreed.

Plan B said:

I was "experimenting" with a similar effect and my fish tank the other day. The glass on the tank is about 1/2 inch thick and the edges are exposed where the panes meet in the corners. Shining a laser at an angle on the edge causes the glass to act as a prism and reflect the beam back and forth inside the glass down the length of the pane. It's a pretty cool looking effect but I'm not positive the fish agreed.

Click to expand...

Interesting effect, I wonder if you can get anything similar by filling in a double-pane window with various fluids. Of course I would consider making that a permanent set-up and put the window back into it's frame

It's like one bigass fiber, but you need the core refractive index to be bigger, so water surrounded by glass won't work, I think water has an index around 1.3 and glass around 1.5~1.6. The closer the indices, the smaller the acceptance angle.

Bluefan said:

It's like one bigass fiber, but you need the core refractive index to be bigger, so water surrounded by glass won't work, I think water has an index around 1.3 and glass around 1.5~1.6. The closer the indices, the smaller the acceptance angle.

Click to expand...

So if it was a sheet of glass surrounded by water, it would work?

Well, I actually had the thought that it might be mineral oil or something other than water. But as I wasn't sure, I only said 'liquid'. But you mentioned the term 'refractive index' and I decided that was the variable I needed for a material, so I looked into it and the most suitable choices are very toxic.

So I'm not really that serious about the window full of water, but this has me wondering about the fishtank full of water.

I think its difficult to find any liquid that has a low enough refractive index to make it work well.

One funny thing is doing the exact opposite: As lab demo we used to fill a small tank with toluele, and then submerge glass objects in it. Since the refractive idices are nearly identical, the glass appears to just vanish. Its especially cool to use something like a measuring beaker with a scale on it - that seems to be just in the middle of the liquid and can keep people puzzled for a while.

Plan B said:

I was "experimenting" with a similar effect and my fish tank the other day. The glass on the tank is about 1/2 inch thick and the edges are exposed where the panes meet in the corners. Shining a laser at an angle on the edge causes the glass to act as a prism and reflect the beam back and forth inside the glass down the length of the pane. It's a pretty cool looking effect but I'm not positive the fish agreed.

Click to expand...

The laser beam stayed in the glass? I had to re-read your post.