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Q & A: Penetration power of light waves and radio waves

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Most recent answer: 10/22/2007
Q:
If radio waves have a lower frequencey on the electromagentic spectrum then opticle light (therefore does opticle light has more energy?), then why can RF waves penetrate solids while the other can not?
- jake (age 14)
canada
A:
Well, that depends on the solid! Neither the radio waves nor the light waves are particularly good at penetrating into conductors, like metals. Light waves are good at penetrating glass, and I suppose the radio waves are too. Saltwater is more or less transparent to light waves but not to radio waves.

On your first question, yes, light in the visible spectrum has more energy per photon than a radio wave. The penetration depth of electromagnetic radiation does not depend (except in some unusual cases) on how many photons there are.

Many solids do not allow light to be transmitted through them because of all the opportunities there are for the light to be absorbed. The rate at which light is absorbed by a substance depends on the availability of energy states of the molecules of the substance which have energy differences equal to the energy carried by the light waves. Glass transmits light because no transitions are allowed between electron states which are stuck to the silicon dioxide matrix and the next energy states up, which allow the electrons to propagage (this requires too much energy). Get the frequency of the light high enough, and the glass will start scattering and absorbing the photons.

Many materials have allowed transitions within the energy range of photons of visible light. Some materials have more allowed states for some energy ranges than others, causing them to be colored. The energy of a single photon in a radio wave often isn't enough to get anything moving in many substances, and the radio wave then just passes through.

Radio waves can often be thought of more classically, in that they create (nearly) static electric fields which can push and shove the electrons around in materials which can allow that. Microwave ovens, for example, use radio-frequency waves to excite vibrational states of water molecules, but also to cause ions to flow around inside the food, resistively heating it.

Tom

(published on 10/22/2007)

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