# Light Frequency.

I've seen that all of the frequencies of everything -light, sound, etc- fits onto the same spectrum. My question is as follows:

What is the lowest/highest frequency at which light stops being light?

Like... A lower frequency would move at the speed of sound but, as the frequency increases, it reaches a threshold at which it shoots off, moving 1,000,000 times faster...

Also: What is classed as light? Is the infrared still counted as light. (I was told that waves at that frequency are what we see as heat but it can be both light and heat, right?)

Correct me if I'm wrong.

Doc Al
Mentor
I've seen that all of the frequencies of everything -light, sound, etc- fits onto the same spectrum.
Light is an electromagnetic wave; sound is a mechanical wave. While both have frequencies, they are very different sorts of things.

My question is as follows:

What is the lowest/highest frequency at which light stops being light?
I don't know what you mean by light stopping being light. Are you asking for the range of the visible spectrum? See: http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2"

Like... A lower frequency would move at the speed of sound but, as the frequency increases, it reaches a threshold at which it shoots off, moving 1,000,000 times faster...
That doesn't happen. All electromagnetic waves travel at the same speed in a vacuum--the speed of light!

Also: What is classed as light? Is the infrared still counted as light. (I was told that waves at that frequency are what we see as heat but it can be both light and heat, right?)
Yes, infrared radiation is an electromagnetic wave just like visible light. See: http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html#c1"

Last edited by a moderator:
Let me rephrase my question:

What is the range of frequencies at which electromagnetic waves can exist?

Also let me ask another question:

Is there a limit on the range of frequencies available to mechanical waves?

One more:

Are mechanical waves dictated by Planck Relation?

What is the range of frequencies at which electromagnetic waves can exist?
The lower range is certainly a frequency of zero. The upper? Thats a tough one, it would have to involve physics beyond the simple $$E=h\nu$$. Its the same as asking is there an upper limit to the energy of a photon. I don't know.

Is there a limit on the range of frequencies available to mechanical waves?
Yes - For sound, the shortest possible wavelength is on the order of the distance between molecules, so the maximum frequency would be on the order of the speed of sound divided by the intermolecular distance.

Are mechanical waves dictated by Planck Relation?
No, the quantum wave is different from the mechanical wave.

Doc Al
Mentor
Let me rephrase my question:

What is the range of frequencies at which electromagnetic waves can exist?
One can speculate that the highest possible frequency would be related to the Planck time, but I don't know much about it. But for practical purposes, there is no limit.

Also let me ask another question:

Is there a limit on the range of frequencies available to mechanical waves?
Yes, as they depend on the mechanical properties of the medium.

One more:

Are mechanical waves dictated by Planck Relation?
Mechanical vibrations are quantized, and the phonon energies obey Planck's relation.