Matter Interactions with EM Radiation

So once you understand about EM radiation and its wave and particle behavior you can use that radiation to probe and find out about the matter around us. Light can interact with matter. Each of the regions affects matter (molecules) in different ways - some subtle, some not so subtle. Let me list for you the regions of the electromagnetic spectrum and also point out how that particular type of radiation effects the atoms and molecules around us (matter). I'll start on the low energy end and finish at the high energy end of EM radiation.

radio waves

You'll be glad to know that radio waves are so incredibly low-energy that they effectively do not interact with matter at all. This is good news for us because we are totally bombarded with radio waves (communication waves) all the time. Radio, television, wifi, cell phones all put out signals. Matter doesn't really do anything. It is one of the reasons that radio waves can travel so far. All those air molecules in the way just don't interact, so the signal can go for miles with little attenuation.

human interaction - There are no harmful effects of radio waves interacting with humans. We've been bombarded with radio waves for over a hundred years. Unless you are "conspiracy guy" those radio waves aren't doing anything harmful to you.


Well now we are starting to get a bit more interaction with matter. Microwaves have just the right amounts of energy to get molecules to rotate. The molecule itself is intact, it just is absorbing enough energy to rotate about an axis. If the microwave is tuned properly we can really get the molecules to rotate. A microwave oven does this with water molecules (sort of... yes, it is more complicated). The bottom line for US is that microwaves are a gentle energetic nudge to cause molecular rotation.

human interaction - Microwaves are fairly harmless to us. I'm assuming you would only be catching a passing stray microwave - like a radar gun. Radar is microwave radiation and it doesn't cook you. Now, if you jack up the power of the beam, like in a microwave oven where it is high power and dose, then yes, you would "cook" just like anything would containing moisture in a microwave.


Stepping up another notch in energy we have IR radiation. This radiation does work much harder on molecular dipoles and gets the molecule to vibrate. Vibrations in molecules are the bending and stretching of bonds at frequencies that match up well with IR radiation. So we say that IR causes molecular vibration. Still no danger of breaking yet - just stretching and bending.

human interaction - Infra red is harmless to us. TV remotes (if not bluetooth) are mostly IR diodes shining IR at your TV receiver. Our eye can't see it - some digital cameras will pick it up though. You really can't do damage by holding on the TV remote and burning yourself. IR is safe. Don't confuse this with what is called an infra-red heating lamp - that is a bit different with a focuses beam of heat we call infra-red because much of its wavelength is in the IR.


Visible radiation is the first one to actually start interacting with actual electronic energy levels within the atom or molecule. Visible light can excite an electron enough to get it to move into a higher energy state. Sometimes we show this excited state with a * or "star" designation. Say the molecule formula is "M" which is in the ground state electron configuration (all electrons in the lowest states possible) - then when a visible photon is absorbed, an electron somewhere in the molecule moves to a higher energy state which is depicted "M*" and we even say em-star. This is also why we perceive colors from all the different types of matter - generally speaking, if white light is coming in, but some of those photons are absorbed (specific wavelengths/colors), then the transmitted light is no longer white. The term we use for this type of energy jump is electronic excitation, the electrons are excited to new higher energy states.

human interaction - Visible light is also a safe radiation for us humans. Heck, we need it to see. In general, normal intensities of visible light are harmless and safe. Jack the intensity up enough though and enter the eye (like a laser) and you will have damage. So caution around intense sources, but overall a safe radiation.


Ultra violet light is just like visible - it causes electronic excitation. However, it is so energetic that a simple jump to a higher level within the atom or molecule starts to go beyond that. The UV photon can provide so much energy that the electron is promoted all the way out of the molecule. This means an electron is ejected and the result is ionization. Although the low energy end of UV typically just excites, the rest of UV (most of it) ionizes. It is for this reason that we refer to UV radiation as ionizing radiation. It is the gentlest version of ionizing radiation, but it still tends to ionize.

One more thing... UV is split into 3 regions: UV-A (320-400 nm), UV-B (280-320 nm), and UV-C (200-280 nm). Ozone and oxygen take out most of UV-B and UV-C.

human interaction - UV is a mixed bag because you can consider each type: UV-A, UV-B, and UV-C and think about its impact. UV will generally only interact or penetrate the skin. This might just lead to a mild sunburn - or, it could be far worse. Interestingly, UV-C is the most damaging (highest energy) but the least dangerous because our atmosphere filters all of it out (thank you ozone and oxygen). UV-B gets filtered out a bit but enough still gets through such that it is the major cause of sunburn and ultimately skin cancer. UV-A is the least energetic but it penetrates the deepest in our skin. UV-A radiation plays a major role in premature skin aging and wrinkle formation. There is also a LOT more UV-A in sunlight than UV-B (about 500 times more). The bottom line again is if UV radiation is hitting you long enough, it will penetrate and damage DNA and can ultimately lead to skin cancer. Sunscreen can help reduce the amount that gets through to your skin (#goodidea).


Picking up where UV leaves off is x-rays. These rays are 100% ionizing radiation. As x-ray photons zip through matter they can interact and when they do they really knock out electrons in a big way. Pretty much any bonding electron will be knocked out to cause ionization. So x-rays are ionizing radiation as well, even more so than UV.

human interaction - x-rays are definitely harmful when there is a sustained dose. Of course they are the ultimate in penetration - they go right through you and many other things. Medical x-rays and screening at airports are small snapshot doses of x-rays and your body can deal with it and recover. Prolonged exposure will ultimately cause internal damage to DNA and other vital molecules.


Gamma radiation is the most powerful radiation that we measure. It is extremely ionizing - knocking out electrons, sometimes multiple electrons, and fragmenting molecules. This is one of the radiation types that persists after a nuclear bomb (x-ray is the other one). So gamma radiation is the king of ionizing radiation.

human interaction - Much like x-rays, if you only take a tiny hit you'll recover. However, if gamma radiation is sustained, it can be very lethal. Avoid completely. Did you watch the mini series "Chernobyl"? It's that. Ionizes everything it contacts - your DNA, your enzymes, you. A terrible way to go as well.

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