The shorter the wavelength, the more easily it’s attenuated, right? The shorter the wavelength, the more easily it’s blocked by matter.
X-ray uses shorter wavelength than visible light. Yet X-ray can be used to see through things that visible light cannot.
I would expect wavelengths longer than visible light to be used to see through matter.
I’m missing something, what is it?
things seem solid yet they are not.
a window screen from a distance at at a certain angle looks solid, yet up close and straight on you see it has holes and passes light.
lots of matter isn’t a dense solid. put a flashlight in you mouth (the bulb end while it is turned on) and in a dark room you will see light through your cheeks.
if the short wavelength x-ray can find a straight path then it goes through. if not some that hits is scattered or blocked.
so it is a matter of degree how much gets through, lots of things change that.
Do X-ray machines use a lot more power than standard visible light illumination devices?
Whence comes the bolded part? That’s probably your problem
Photons of light at shorter wavelength have more energy than photons of light at longer wavelength. X- and gamma-ray photons have more energy than ultraviolet photons, which have more energy than visible photons, etcetera. This is why you don’t want to stand in front of a gamma ray source or an x-ray source. The high energy photons can cut chemical bonds, including DNA, and can do nasty things to biological tissue. Hard ultraviolet photons will give you nasty burns and afftect your eyes. Long wavelength UV has enough energy to kick luminescent systems to higher energy levels, and even after losing some energy to the surrounding matrix, still has enough energy to get released as a visible photon. That’s how BlackLights work, and so forth.
higher energy photons can breeze by things that will absorb lower energy photons, hence X-ray phtos. Even ultraviolet photons have enough energy to get by alkali metal absorption, so alkali metals are, surprisingly, transparent to UV light.
x-rays are done for many purposes. the type and purpose will determine the power used.
an x-ray beam might be a few to many thousands of times more powerful.
I’m guessing the bolded part stems from extrapolating the well-known phenomenon that low frequency sounds propagate farther than high-frequency sounds. (See the graph of absorption coefficient versus frequency in Brandon Enright’s post.) This overgeneralization ignores the fact that sound waves travel by physically displacing the molecules of the intervening medium, whereas no such medium is required for electromagnetic waves.
Sunlight irradiates the surface of the earth at about 1000 watts per square meter. What is the intensity of the x-ray radiation used in (for example) a chest X-ray?
The sun emits at a very broad range of wavelengths, some of which are absorbed by our atmosphere. Is the 1000 watts per square meter specifically a measurement of naked-eye visible light?
The 1000 W/m^2 is the sea-level value, i.e. after it’s passed through the atmosphere. And IIRC, it’s for the entire spectrum, not just visible wavelengths.
yep, here:
They do make the point that most of that power is in the visible wavelengths (the peak is in the green part of the spectrum).
I was extrapolating from radars and radios. Low frequency radars and radios tend to have longer range than high frequency ones.
Low frequency communications can penetrate using the electromagnetic spectrum can penetrate deep into the sea and rock (Extremely low frequency - Wikipedia) whereas EHF have much shorter range Extremely high frequency - Wikipedia
Electromagnetic waves of different frequencies have radically different properties. The biggest difference between x-rays and visible light is that x-rays are ionizing radiation; that is, atoms cannot absorb and re-emit x-rays by raising and lowering the energy of their electrons- the x-rays are too powerful. Instead, either an x-ray photon passes completely through a substance or it knocks an electron away from an atom altogether (the ionizing part). To vastly oversimplify things, x-rays couple more poorly to atoms than visible light does.
So, when an x-ray shows a bone, that means it knocked electrons off the atoms of those bones?
What happens to the x-ray photon when it knocks an electron away from an atom^
Yes. Making an image only works if there is some selective absorption/scattering and some transparency. That makes the white and black (and gray) parts.
If you “shine a light” (e.g. x-rays) on something and nothing gets through, there is no image. If all of the “light” passes through equally well, again, no image.
When a medical x-ray of a patient’s bone is made, the technologist chooses an appropriate technique (selection of intensity and beam enrergy) to match the expected absorption frequency of bone. This is actually significantly adjusted by experience (what gives a good picture), and bones are NOT the only thing that need to be shown. There are other compromises, with adjustments based on the patient’s body size.
But bottomline, if x-rays are not absorbed or scattered in the patient’s body, then no images are going to be formed.
I like this way of saying it.
The longer range of lower frequencies is much more a factor of whether they are refracted by the atmosphere. The reason you can hear a relatively low frequency AM station from halfway across a continent is because the signal is bouncing between the ground and the ionosphere. Meanwhile at just slightly higher frequencies the signal goes straight through the ionosphere and all communications are therefore limited to line of sight limited by the Earth’s curvature.
ELF penetration into water or rock is a different phenomenon.
What you’re missing is that absorption of light can’t really be well described by a continuum model of matter.
For any kind of transmitted energy to be absorbed, you need the absorber to respond to the excitation and absorb the energy, which generally means there has to be some internal mode of excitation of the absorber that can suck up the energy. For example, a sound wave is absorbed by exciting some wiggling of the same frequencies in an absorber. The more wiggling motions of the absorber there are, the better chance there is that some of them will match the incoming sound wave, and the better absorption you’ll get. Now, for sound, a continuous description of matter is very reasonable. A continuum generally has many more high-frequency modes of vibration than low, so indeed you would find that high-frequency sound would be absorbed better and faster than low frequency. Hence distant thunder is low, elephants can be heard over miles while parakeets only over feet, et cetera.
This continuous to hold reasonably true for electromagnetic radiation provided you are still talking about big wavelengths, e.g. radio. So longer radio waves do indeed go further and are more penetrating, because there are fewer modes of vibration of matter at that wavelength that they can excite, and transfer their energy to.
But when you get to very small wavelengths – microns and smaller – then it starts to matter a lot that matter is not continuous, but made of atoms and molecules, and the particular atoms and molecules you’ve got determine what kinds of internal excitation your EM radiation can excite.
It turns out that microwaves tend to hit the rotational motion of certain molecules, and a lot of materials have molecules that can rotate. But liquids more than solids, which is why water heats up faster than bread in your microwave (in addition to the fact that the oven is partly tuned to the frequency of water molecule rotation). Go a little shorter, into the far infrared, and you start to get into some vibrational motions of molecules, but only big ones, so far infrared is pretty penetrating. That’s why it’s of interest in astronomy – sees through light years of dust. In the near infrared, you hit vibrational motion in nearly anything, so matter is strongly opaque in the near infrared. You can’t even see through glass very easily. Shorter still, you hit the visible. Now, interestingly, visible light tends to excite very little. It’s too high frequency for vibration, not high enough for anything else. So matter is often surprisingly transparent in the visible – there just aren’t many internal excitations that can absorb the energy. You might say, whoa, amazing then that the Sun puts out most of its radiation in the visible – how very convenient for seeing creatures such as us! Intelligent design! Well, maybe, except that if the Sun put out most of its radiation somewhere else, it’s unlikely life would even be possible, because the energy would be too easily dissipated (if too low wavelength) or blast apart any complex molecules to readily (if too high).
Get a little shorter, and ultraviolet light tends to start exciting electronic transitions, electrons hopping from one part of the molecule to another (roughly speaking). Matter starts to become opaque again in the UV. Also, this hopping of electrons around can easily lead to rearrangements of chemical bonds – i.e. chemical reactions – which is why UV light is destructive of materials – plastics, glues, wood, leather, and flesh itself.
Shorter still, into the soft X-ray, and again matter becomes fairly transparent, because only the heaviest atoms have electrons at such low energies that they can absorb the energy in an X-ray. So X-rays tend to pass unmolested through light atoms, and only get absorbed by the heavier ones.
Keep going far enough, into the hardest of X-rays and early gammas, and you start being able to excite vibrations (so to speak) of atomic nuclei, which are exceedingly fast indeed, and then again matter becomes somewhat more opaque.
In general, the absorption of matter of EM radiation is a very complex function of frequency, and it rises and dips in ways that are not easily described and which depend sensitively on the precise nature of the matter – its chemical and even nuclear constitution. In fact, this is how we learn a great deal about matter: by shining light on it, or through it, recording the spectrum of absorption, and using microscopic modesl to try to explain the spectrum in terms of what kinds of motion is possible at the microscopic level.
Thank you very much for the long answer, Carl Pham.
Gamma rays mess up the atom’s nuclei, right? Is there anything higher frequency than gamma rays? Looking at Wikipedia it appears that not but I may be missing something.
What is the smallest/least dense thing that would be excited/start to be opaque to a 1Hz transmission? Is it possible to go below 1Hz and have wavelengths of more than 300 000KM?
The shortest visible wavelength is violet, right? The Doppler effect is almost always portrayed as giving a red shift to objects travelling away and a blue shift to objects closing in. Should it be a violet shift?
Seems to me long wavelengths would be more easily attenuated. Can’t a small animal squeeze through small spaces where a larger animal is attenuated???
You really just have to go back to Carl Pham’s post.
The bottom line is that if the frequency of the electromagnetic (EM) wave resonates with the medium (or some component of it), it will interact with it (or couple to it), and deposit energy into it (or be absorbed).
To be honest, I’m not sure how Compton scattering plays into this discussion- I’m thinking it is a high energy phenomenon and does not require much frequency matching.