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can someone please explain me what we mean by 'high-level' or 'low-level' in electromagnetic radiation? for example, it is believed that high-level microwave radiation is harmful to human but not the low-level one.

what is this level here we are talking about?

if the frequency and wavelength is same, how high-level and low-level radiation differs?

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  • $\begingroup$ Seems to refer to exposure, either because Intensity is high or because the time receiving it is longer. The effect would be the Dose or amount of energy deposited in the tissue. $\endgroup$ – rmhleo Sep 19 '14 at 11:55
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It is believed that high-level microwave radiation is harmful to human ...

It is not just believed but well known that extremely intense microwave radiation will cook people. We use microwaves to cook meat, after all, and a good portion of our bodies is in the form of meat. Lesser intensities can cause survivable burns, even lesser intensities might cause cataracts and possibly sterility. Below that, microwave radiation is generally safe. It's non-ionizing.

What is this level here we are talking about?

That would be the specific absorption rate, which is the rate at which the human body (or some part of the human body) absorbs non-ionizing radiation. The specifics of how this is calculated and the threshold levels vary from country to country.

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An important factor in determining how much energy electromagnetic radiation carries is its intensity, which is just the power per area. In fact, it can be found in the wikipedia article that intensity is sometimes taken to be synonymous with 'level' (even though it is not really correct), so surely this is the right variable to look at.

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I agree with @DavidHammen here, the 'high' and 'low' most likely refer to intensity. A microwave oven works by causing water molecules to oscillate (i.e., shake back-and-forth), which results in the generation of energy (i.e., heat) due to intermolecular "friction." Every molecule in the universe has a resonant frequency.

Think of "rocking" on a swing set to understand this concept. If you move your mass correctly, you are able to increase the amplitude (i.e., displacement from the bottom where the swing sits when at rest) of your oscillation. This is one example of driving a system near a resonant frequency. Your body does the work by shifting your mass relative to the swing seat and the amplitude of your oscillations increase if you do this at the "correct" rate. To test this further, try to "rock" twice as fast as you normally do to get large amplitudes. You will find that you don't get much out of this effort.

Microwave ovens do work on water molecules by adding a driving force (through electromagnetic waves/radiation) that is near the resonant frequency for those molecules. Well, technically most microwave ovens drive water at a factor of 10 away from the molecules resonant frequency.

Recall that you are made mostly of water, so your body would react strongly (in a bad way) if exposed to high intensity (e.g., > few hundred watts, whereas a microwave oven is at 1000-1400 watts). If you were somehow put into a microwave oven, you would not be happy, to say the least.

Side Note 1: Ionizing radiation is any type of radiation (e.g., electromagnetic, particle, etc.) that deposits enough energy to either dissociate a molecule, free an electron from a molecule/atom, or split an atom (i.e., fission).

Side Note 2: The energy of any given photon (i.e., the equivalent of a particle for light [= one type of electromagnetic radiation]) is defined by: $$ E = h \nu = \hbar \omega = \frac{h \ c}{\lambda} $$ where $h$ (= 2 $\pi \ \hbar$) = Planck Constant, $\omega$ (= 2 $\pi \ \nu$) = frequency, $c$ = speed of light, and $\lambda$ = wavelength. Since $h$ is constant, then each photon's energy is just determined by its frequency. Microwaves range from roughly 3$\times$10$^{8}$ - 3$\times$10$^{11}$ Hz (= 300 MHz to 300,000 MHz), though these ranges aren't really set in stone. Regardless, $h$ ~ 10$^{-34}$ J$\cdot$s or ~ 10$^{-15}$ eV$\cdot$s which means that microwave photons carry ~ 10$^{-25}$ - 10$^{-22}$ joules or ~ 0.000001 - 0.001 eV. For reference, the first ionization energy of atomic hydrogen is ~13.6 eV, thus why people say that microwaves are non-ionizing radiation.

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