# Is the frequency of light restricted?

What are the factors that limit the frequency of light? Can it have wavelengths ranging between zero and infinity?

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Well a finite universe probably has finite energy, so if E=h*nu still works then that's an upper limit, if it has finite spatial extent then you have a lower limit (for half a wavelength). –  Sean D Jul 27 '14 at 22:25
If production of high frequency light is possible, then we can make develop some future weapons. –  Afrish Khan Jul 28 '14 at 6:36
But still we don't know about the finiteness of the universe –  Afrish Khan Jul 28 '14 at 6:38
"High frequency light" would be X-rays and gamma radiation. We can already produce that, the thing we can't do is aim it. And we're trying very hard to achieve that. Don't bother with weapons research, the chip business needs it now. ASML is fighting the problem that lenses don't work anymore at the extreme UV wavelengths. –  MSalters Jul 28 '14 at 8:16

## 4 Answers

The electromagnetic spectrum does range between (almost) zero and (almost) infinity. It's just that your eyes are sensitive to a very small part of it (from about 380 nm to about 800 nm).

At the lowest frequencies, it becomes difficult to recognize the signal from background fluctuations.

From this site: "Gamma-rays are detected by observing the effects they have on matter." So the upper end of the EM spectrum is probably above 300 EHz, but there are no EM generators at those frequencies.

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The Wikipedia says that it ranges from 300 EHz to 3 Hz. But my question is why not it extend even further? –  Afrish Khan Jul 27 '14 at 18:01
Not about the detection of the waves of higher and lower frequency,but I'm inquiring about its presence anywhere in the Universe. Can they occur in nature? –  Afrish Khan Jul 27 '14 at 18:14
If they cannot be detected, how can you know whether they occur? –  rob Jul 27 '14 at 18:28
I'm interested in what you mean by "stray capacitance in the detectors" when you are trying to detect, say gamma rays. I thought at these frequencies the detection mechanism isn't through an electrical circuit in the same way it is for, say, radio waves. –  Michael Jul 28 '14 at 2:53
I was thinking of electronic circuits (capacitance between lines), but you are correct in that some detectors don't use electronics (i.e. film and neutrino detectors) for the initial detection. The detector would be specific for the task, like for a gamma ray burst, you want to know the location it originated from and the intensity, you may not need to know the spectrum distribution (although it may be important). I will need to research on how very high frequencies of the EM are detected. –  LDC3 Jul 28 '14 at 3:15

Do keep in mind that the frequency of light is reference frame dependent. So, for example, the cosmic background microwave radiation would appear as a concentrated gamma radiation source 'in front' to an observer with ultra-relativistic speed relative to the CMB.

In other words, light emitted from a body of a particular frequency in that body's frame of reference, could have arbitrarily low or arbitrarily high frequency in relatively moving reference frames.

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Do Relativity have impacts on frequency of the waves? –  Afrish Khan Jul 28 '14 at 6:30
@AfrishKhan It's a bit more complicated than that. Think about a car wheezing past by you - is the frequency of the sound changing, or is it just your perception that's changing? The truth is, in a relativistic universe, there is no preferred referential frame, so you can't really say the objective frequency, because there is no objective frame of reference. Now, speed of light is always the same, however, the energy of a photon changes. So yes, if you move faster relative to a photon, it will have a higher energy and frequency, as far as you can tell. –  Luaan Jul 28 '14 at 7:53

Theoretically, the shortest wavelengths of light would be limited by the Planck length, at some point the space 'closed' by the wavelength would be so small that gravitational effects would dominate, in the same way that black holes can bend light passing near their event horizon at very small scales the wavelength would be so small that it might be at the centre of its own super-tiny black hole and so be unable to escape.

We don't have much of an understanding of what happens at these scales of space though, the Planck length is 10-20 times smaller than the diameter of a proton and what gravity does at this extreme is one of physics' central mysteries.

I'm not too sure about what the limits of the lowest frequency / longest wavelength are, I suppose that the longest wavelength would be limited by the age and size of the universe, as far as we know the Universe has an age and the speed of light has a fixed upper limit so perhaps the longest possible wavelength is linked somehow to whether the Universe is expanding or not and whether the big bang really was the start of all quantum events.

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aren't there limitations or an upper bound due to spontaneous pair production? –  Michael Jul 28 '14 at 2:54

The lowest frequency limit is provided by the size of the universe. If we could make an antenna that size, the frequency would be "very close" to zero. The highest EM frequency limit is provided by the smallest antenna we could make. I believe that would be the size of a hydrogen atom, giving us, $$F_u = \frac{2.997x10^8}{6.28x5.29x10^{-11}} = 9.02x10^{17}Hz$$

Note: These are theoretical limits, the current "real" limits are as reported on Wikipedia.

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## protected by Qmechanic♦Jul 28 '14 at 13:05

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