QUESTION #175

# If gamma rays can travel through walls, and radio waves can travel through walls, and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum?

Asked by: Nakul Jeirath

It's a Goldilocks and the Three Bears example of this radiation is too big and slow and this radiation is too small and fast.

Gamma rays, visible light, and radio waves are all examples of electromagnetic radiation, and you're right that radio waves are on one end of the spectrum (with a wavelength of meters), gamma rays are on the other end (with a wavelength of about 10-12 meters) and visible light is somewhere in between (with a wavelength of around 5 x 10-7 meters).

Three things happen to EM radiation when it encounters a barrier. It can bounce (reflectance or scattering), pass through (transmittance), or just plain stop (absorbance).

The intensity of radiation transmitted depends on several things:
1. The wavelength of the radiation
2. The intensity of the radiation hitting the barrier
3. The chemical composition of the barrier
4. The physical microstructure of the barrier
5. The thickness of the barrier
Your question has to do with #1, but keep in mind that powerful enough visible light (many times brighter than sunlight) would be able to pass through the walls of your home so you could see it, and even a weak light can shine through a paper-thin wall or a wall made of glass.

Now as for your question, imagine an ant and an elephant standing next to each other. The ant doesn't know the elephant is there because it's too big and the elephant doesn't know the ant is there because it's too small.

EM waves interact with matter in a similar way: on their own scale of size and time.

The reason why visible light can't travel through walls as easily as gamma rays or radio waves is because, to the visible light, there's something 'there' on a similar scale of length (wavelength) and time (frequency) that the gamma rays are too small and fast to interact with and the radio waves are too large and slow to interact with.

The gamma rays just pass through the spaces between and within atoms and molecules. They're small enough to do that. If your walls were made of pure lead, they'd have a little more trouble.

The radio waves on the other hand are large enough to ignore the walls because they're too thin. But if you've ever tried to listen to a radio in a tunnel or a cave, you know that radio waves have trouble penetrating through meters of earth.

Visible light interacts with many solid materials because the wavelengths are the right size to be scattered by structural elements within those materials like microfibers and cracks and porous spaces. Well-cut glass or diamond don't have these structural elements at this length scale so they look clear unless they're shattered or rough.

So scattering due to physical elements of the right size is a major reason, but another factor contributes too.

Visible light also interacts with pigments in the paint on surface of your wall. This occurs because the pigment molecules have chemical bonds within them that only vibrate at certain frequencies. When visible light at that frequency hits the molecule, it's absorbed. The frequency of gamma rays is far too high and that of radio waves is far too low to interact with these kinds of vibrations.
Answered by: Marc Shelikoff, B.S., ChemE Grad Student, MIT