By turning the bulb through a right angle the electroscope which had previously indicated a maximum of intensity indicated a minimum, and vice versa. The position of the bulb when the vertical secondary beam attained a maximum of intensity and the horizontal secondary beam a minimum was that in which the kathode stream was horizontal, the maximum and minimum being reversed when the kathode stream was vertical.
By turning the bulb through another right angle, so that the kathode stream was again horizontal but in the opposite direction to that in the other horizontal position, the maximum and minimum were attained as before. For a number of years, his results were the strongest evidence of the relationship between light and X-rays; as noted by the Braggs in their book, X-rays and Crystal Structure ,.
Undoubtedly the strongest evidence — up to the present time — of the similarity of nature of light and X-rays was supplied by the discovery of a form of polarisation of the latter rays. Barkla showed that the X-rays issuing from a bulb and impinging upon matter were less scattered by the matter in a direction parallel to the stream of cathode rays in the bulb than in directions at right angles to the stream.
A , Another great post! I can totally understand why they leave this sort of thing out of most textbooks, because otherwise it would take too long to teach the material. But an unfortunate side effect is that the non-specialist public who may have had one or two science courses in college, but never went back afterwards to look up all these experiments are largely unaware of how much experimental evidence there is for the things in science textbooks. Perhaps it would help if if introductory science classes concentrated more on this kind of experimental evidence, and less on teaching the distilled end results without any supporting information.
Wade: Thanks for the comment!
I think it would be helpful, especially because it could give students a better feeling for how scientific discoveries are made. A really nice post. I would like to make a suggestion. I would like to make it clear that the article is excellent. This is the only bit I noticed could be phrased better. I guess you mean that tubes of this design can only produce X-rays up to keV.
For diagnostic radiology tubes have a maximum around keV for reasons of usefulness, contrast decreases at higher energies due to the change from photoelectric to Compton interactions. This is currently common around the world for treating prostate cancer, for example. The reason I think it is worth noting this is that it is a common misconception that some sort of useful distinction between types of photons gamma, X-ray, … can be made based on energy.
When usually it is the mode of production that is the most important difference. CF: Thanks for the comment! It makes sense, though, that there is more to a photon than just its average energy, for instance bandwidth. Random but interesting digression: it seems that Nikola Tesla may have actually taken the true first X-Ray photographs. Joshua: Interesting! A similar thing happened with Rutherford vs.
Rutherford, of course, independently came up with the idea after the famous gold foil experiment. I came across this while trying to find out whether X-rays are circularly-polarized radiation like gamma rays are, I believe or plane-polarized radiation, or elliptically-polarized radiation. I learned that they are usually unpolarized, a mixture of plane-polarized radiation containing all possible orientations of the plane of polarization. I expect that the manner of generation of the radiation determines which type of polarization is present.
Physicists like the idea that electromagnetic radiation has a characteristic energy value determined by its frequency; the circularly-polarized version behaves like this. I wonder why he used air as scatterer, because the scattering of X-rays in air is very small. It is surprising that he could detect it with those means. I also wonder what is the grade of polarization of the primary beam.
Right or wrong, in the field of radiation protection, medical physics, etc it is always assumed that polarization effects in ordinary X-rays are totally negligible in practice. Thanks for the comment, and apologies for the delay in replying! Not an expert on x-ray tubes, but my impression is that the amount of x-rays coming off of the anode perpendicular to the electron motion is very small. Diagnostic X-Ray tubes use electrons up to around keV.
These tubes produce X-Rays at 90 degrees to the incomings electrons. I think this is because X-Rays produced in-line with the electron beam tend to get attenuated by the anode again. Most efficient way is to produce them at 90 degrees. The anode angle may vary depending on what the tube is for mammography tubes are a bit different, I think. Once you get up to linacs electrons 4MevMeV for therapy the x-rays are produced in line with the incoming electrons.
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Skulls in the Stars. Skip to content. Herbert S. Not being able to help myself, I tracked down the original source… As always, a little background will be helpful. The cathode rays impinged upon the broad end of the tube. From G. Kaye, X-rays , Longmans, Green and Co. An anotated picture of a tube from the early s is shown below: A Cossor bulb with automatic softening device and fin radiator for cooling anticathode.
Adapted from G. It is worth noting, for future use, that the X-rays produced at the anticathode more or less spread out in all directions; the following figure illustrates the intensity of X-rays as a function of the angle from the normal to the anticathode: Graph showing distribution of X-rays, the cathode rays being normally incident on an anticathode. Electromagnetic waves are transverse waves, however, which means that the electric and magnetic fields oscillate in directions perpendicular to the direction of motion: The term polarization 1 is used to describe the transverse behavior of the electric field.
However, as noted by Thomson, No evidence of any polarisation of the rays has been obtained; the opacity of two crystals of tourmaline or of herapathite, placed one on the top of the other, is the same when the axes of the crystals are crossed as when they are parallel. This oscillation, in turn, causes the electrons to give off the secondary X-ray radiation: The electric field produced must oscillate in the same direction that the electron vibrates.
Looking back at how X-rays are produced in an X-ray tube, however, Barkla realized that there was already a way that polarized X-rays are produced, if they are in fact electromagnetic waves: A consideration of the method of production of X-rays, however, leads one to expect partial polarization from the anticathode in a direction perpendicular to the axis of the kathode stream. In other words, electrons come to a screeching halt at they hit the anticathode; in a plane perpendicular to their line of motion, X-rays should be excited which are polarized: So how did Barkla take advantage of this?
His diagram of his experimental setup is shown below: The X-ray bulb, D, emanates rays in all directions. This is illustrated schematically below: The electroscopes used for detection are worth mentioning. So what did Barkla see? In his words, As the bulb was rotated round the axis of the primary beam there was, of course, no change in the intensity of primary radiation in that direction.
For a number of years, his results were the strongest evidence of the relationship between light and X-rays; as noted by the Braggs in their book, X-rays and Crystal Structure , Undoubtedly the strongest evidence — up to the present time — of the similarity of nature of light and X-rays was supplied by the discovery of a form of polarisation of the latter rays. Share this: Tweet. Like this: Like Loading This entry was posted in History of science , Physics. Bookmark the permalink. June 8, at am.
June 8, at pm. ColonelFazackerley says:. June 13, at am. Once you look into astronomy, X-ray photon energies can, of course, get even wilder. June 15, at pm. Joshua says:. July 6, at pm. Marjorie says:. March 27, at pm. But what I'm trying to say is, at any given moment, you don't know what direction the electric field's going to be hitting your eye at from a random source.
It could be in any direction.
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- Barkla shows that x-rays have polarization () | Skulls in the Stars?
So this is not polarized. This diagram represents light that is not polarized. At some point, the field might be pointing this way, at some later point it's this way; it's just random. You never know which way the electric field's going to be pointing. Whereas these over here, these are polarized. So how could you polarize this light? Let's say you wanted light that was polarized. You were doing an experiment. You needed polarized light. Well, that's easy. You can use what's called a polarizer. And this is a material that lets light through, but it only lets light through in one orientation, so you're going to have a polarizer that, for instance, only lets through vertically polarized light.
So this is a polarizer. These are cheap: thin, plastic, configured in a way so that it only lets light through that's vertically polarized. Any light coming in here that's not vertically polarized gets blocked, or absorbed. So what that means is, if you used this polarizer and held it in between your eye and this light bulb, you would only get this light. All the rest of it would get blocked.
Or you could just rotate this thing and imagine a polarizer that only lets through horizontal light. Now it would only let through light that was this way, and so you would only get this part of the light. Or you could just orient it at any angle you want and block everything but the certain angle that this polarizer is defined as letting light through.
So you can do this. And once you hold this up, you get polarized light, light that's only got one orientation. So that's what polarization means. But why do we care about polarization? Well, let me get rid of this for a minute. You've heard of polarized sunglasses. So imagine you're standing near water, or maybe you're standing on ice or snow or something reflective. There's a problem. Say the sun's out.
It's shining. It's a beautiful day -- except there's going to be glare. Let's say you're looking down at something here on the ground. It's going to get light reflecting off of it from just But it also gets this direct light from the sun. So it gets light from reflected off the clouds and whatever, whatever's nearby, ambient light. And there's also this direct sunlight. That's harsh. If that reflects straight up to your eye, that hurts. You don't like that. It blocks our vision. It's hard to see, it's glare. We don't want this glare.
So what can we do? Well, it just so happens that, when light reflects off of a surface, even though the light from the sun is not polarized, once it reflects, it does get polarized or at least partially polarized. So this surface here, once this light reflects, it's coming in at all orientations. You got electric field And when it reflects, though, you mostly get, upon reflection, the direction of polarization defined by the plane of the surface that it hit. So because the floor is horizontal, when this light ray hits the ground and reflects, that reflected light gets partially polarized. This horizontal component of the electric field is going to be more present than the other components.
Maybe not completely. Sometimes it could be. It could be completely polarized, but often it's just partially polarized. But that's pretty cool, because now you know what we can do. I know how to block this. We should get some sunglasses. We put some sunglasses on and we make our glasses so that these are polarized. And how do we want these polarized? I want to get rid of the glare.
Barkla shows that x-rays have polarization () | Skulls in the Stars
So what I do is, I make sure my sunglasses only let through vertically polarized light. Here's some polarizers. That way, a lot of this glare gets blocked because it does not have a vertical orientation, it has a horizontal orientation. And then we can block it. So that's one good thing that polarization does for us, and understanding it, we can get rid of glare. Also, fishermen like it because, if you're trying to look in the water at fish, you want to see in through the water, you want to see this light from the fish getting to you.
You don't want to see the glare off of the sun getting to you. So polarized sunglasses are useful. Also, we can play a trick on our eye, if we really wanted to. You could take one of these, make one eye have a vertical orientation for the polarization, have the other eye with a horizontal Well, the reason our eyes see 3D is because they're spaced a little bit apart. They each get a different, slightly different image. That makes us see in 3D.
We can play the same trick on our eye if we have the polarization like this. If light, if some of the light from the movie theater screen is coming in with one polarization, and the other light's coming in with the other polarization, we can send two different images to our eyes at the same time.
If you took these off, it'd look like garbage because you'd be getting both of these slightly different images, it'd look all blurry. And it does.
If you take off your 3D glasses and look at a 3D movie, looks terrible, because now both eyes are getting both images. But if you put your glasses back on, now this eye only gets the orientation that it's supposed to get, and this eye only gets the orientation that it's supposed to get, and you get a 3D image. So it's useful in many ways. Let me show you one more thing here.
Let's come back here.
Polarization of X-Rays
This light was polarized vertically. So that's called linear polarization. Any time Same with these. These are all linear polarization because, just up and down, one linear direction, just diagonal. This is also linear. All of these are linear. You can get circular polarized light. So if we come back to here, we've got our electric field pointing up, like that. Now let's say we sent in another light ray, another light ray that also had a polarization, but not in this direction. Let's say our other light ray had polarization in this direction, so it looks like this, kind of like what our magnetic field would have looked like.
But this is a completely different light ray with its own polarization and its own magnetic field.
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So we send this in. What would happen? Well, at this point, you'd have a electric field that points this way. At this point, you'd have a electric field that points that way. What would your eye see if you were over here? Let's see. If I draw our axis here. All right, when this point right here gets to your eye, what am I going to see?
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Well, I'm going to have a light ray that's one part of a light ray.