# HELP! wavelength and object penetration

1. Jan 26, 2006

### chris4434

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it is generally accepted that 800 mhz frequencies (longer wavelengths) travel farther and penetrate buildings better than 1900 mhz frequencies (shorter wavelengths) I have looked for hours on the Internet trying to find a scientific explanation for this, but i cant find one. does anyone know exactly what makes longer wavelengths penetrate objects better than shorter wavelengths? any help would be greatly appreciated because i need this ASAP for a project. thanks

2. Jan 27, 2006

### vanesch

Staff Emeritus
I don't think that there is a general reason ; it just happens that these higher-frequency modes are more absorbed by the specific structure of matter around us in buildings and so on.

3. Jan 28, 2006

### pseudovector

Consider a massless spring attached to a point mass. If you force it to oscillate at an arbitrary frequency other then its resonance frequncy, then the spring will resist the external force and it'll dissipate the energy very fast. However, if you force it to oscillate at a frequency very small compared to the resonance frequency, then at any given moment the spring would be in equilibrium and the energy will dissipate slower.
In many ways, solid matter behave as though it's made of many pointlike masses connected by tiny springs, so obviously slow changing waves (low frequency, high wavelength) will penetrate more then fast changing waves (high frequency, low wavelength) since their energy dissipation is lower.
This model is good as long as you are dealing with wavelegnths far greater than the dimensions of an atom (and the radiofrequencies in question absolutely do qualify). Notice, however, that when the wavelength is not much larger then the atoms, then the masses connected by the springs are no longer pointlike, and this model doesn't work anymore.

4. Jan 29, 2006

### Claude Bile

There is a simpler explanation - diffraction.

Longer wavelengths diffract (bend) more when passing through buildings. Obstacles therefore block less radiation from nearby emitters.

To illustrate this, consider the extreme case of light. Light has a frequency of hundreds of Terahertz (with a wavelength of hundreds of nanometres) and thus is unable to diffract around obstacles several metres in size (If it did, we would be able to see around corners). Compare this to sound waves, these waves typically have wavelengths in centimetres, thus they are able to diffract through doorways etc. This is why it is possible to hear conversations several rooms down the hallway.

Note that GHz frequencies have wavelengths in centimetres also.

Claude.

5. Jan 30, 2006

### skywolf

what if you were talking about the difference between a particle eg alpha and a ray eg gamma, could you figure out penetration depth comparing just the energies?

6. Jan 30, 2006

### pseudovector

You need more than the energy of the particle to calculate the penetration depth. Other propetries of the particle such as electric charge play a key role in determining the penetration depth.