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mobile phones run on mirowaves, microwaves on account of having larger wavelength dan visible light have less penetrating power....so how is it that visible light does not pass through walls bt microwaves do(you can talk on a mobile in a closed room)
Comments (3)
Different types of radiation interact with matter in different ways. Radiation will only interact with matter that has energy transitions equal to the energy of the radiation. The energy of radiation is proportional to frequency. If there is no available energy transition, then radiation will not interact.
Visible light is opaque when it is absorbed by an atom's electrons. It transitions the atomic electrons to a higher energy level. If the atom does not have an energy transition equal to the frequency of the visible light, then the matter is transparent.
Infrared light is absorbed by molecular vibrations, which are only of certain energy transitions. If there is no energy transition equal to the light energy, infrared light will pass through.
Microwave radiation is absorbed by molecular rotation. Your microwave oven sees water as opaque because the change in energy of water rotation is equal to the energy of microwaves emitted by your microwave. It sees ice as transparent because ice doesn't have this rotational energy level.
Radio waves, on the other hand, aren't really absorbed by any atomic or molecular energy levels (in a dielectric) as far as I know. That is why it passes through walls.
Microwave Interactions
The quantum energy of microwave photons is in the range 0.00001 to 0.001 eV which is in the range of energies separating the quantum states of molecular rotation and torsion. The interaction of microwaves with matter other than metallic conductors will be to rotate molecules and produce heat as result of that molecular motion. Conductors will strongly absorb microwaves and any lower frequencies because they will cause electric currents which will heat the material. Most matter, including the human body, is largely transparent to microwaves. High intensity microwaves, as in a microwave oven where they pass back and forth through the food millions of times, will heat the material by producing molecular rotations and torsions. Since the quantum energies are a million times lower than those of X-rays, they cannot produce ionization and the characteristic types of radiation damage associated with ionizing radiation.
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It is basically to do with whether an object can absorb energy at the frequency of the wave . Visible light has a frequency in a range that atoms and molecules in solids interact with very strongly. There are lots of electrons which can absorb the energy of a visible light photon, so most things are opaque.
Where as radio waves have a very low frequency and to absorb them efficiently you need charges which can move at this frequency. Because this is low it means that to absorb significant amounts of energy the charges have to be able to move a long way - energy = force * distance if something is vibrating 1014 times a second this distance is going to be a lot shorter than 100 000 times a second, hence large pieces of conductor are very good at absorbing radio waves.
This is a pre-quantum mechanical way of thinking about it, but it all converts nicely into quantum mechanics if you do enough maths. A large piece of metal will have lots of energy levels in the radio part of the spectrum so it can absorb radio waves well, a lump of plaster doesn't.