I would argue that there is no such thing as visible light – or at least that the term visible light is a meaningless one.
Light is part of the electromagnetic spectrum which is describes electromagnetic radiation by its wavelength. An electromagnetic wave has both electric and magnetic field components. What is really very interesting is that depending upon the wavelength of the field the electromagnetic radiation has very different properties and we give it a different name.
When the wavelength is very long, the radiation is radio waves or micro waves. When the wavelength is very short, the radiation is x-rays or gamma rays. There is a narrow range of wavelengths (from about 360 nm to about 780 nm – a nm is 0.000000001 of a metre) to which our eyes are sensitive. Because we can literally see this radiation we call it light. I still find it amazing that light, x-rays, radio waves, and microwaves are all essentially the same thing (electromagnetic radiation) with just a change in the wavelength!! However, my point for today is that light is radiation that is visible – to talk about visible light would be bizarre since by its very definition light is visible. Technically, visible light is a pleonasm; pleonasm is a word derived from the Greek word “pleon” meaning excessive. Other examples of pleonasms – easily confused with oxymora – include the phrases end result and invited guests.
Hi Stephen,
There are many problems with the definition of “light” as a visible part of the electromagnetic radiation. First of all we don’t really see all of the “visible” spectrum. One can prepare two light beams with slightly different spectral content which will be indistinguishable for a human. By definition then we would say that the spectral components which present in one beam and are missing in the other one are not “light” since they are invisible to us. Then there are colour blind people with their own definition of light. There are also tetrachromats. And after all human retina seems to be sensitive to the UV radiation (aphakia). So it is just very impractical to base the definition on human abilities. Light has existed before humankind and will be there after we will disappear.
Another problem would be the light of a very low intensity. If there is just one photon per hour in the beam then it is certainly invisible to us and should not be called “light”. But this single photon will behave exactly like normal “visible” light. It will be properly reflected, refracted, and detected by a good detector.
A convenient definition of light should be human-independent. In fact, I would say that it is not light exists because we see it, but it is other way round: we are sensitive to certain spectrum of electromagnetic radiation because there is “light” – an electromagnetic radiation which interacts with ordinary atoms in a certain way such that the medium which surrounds us is transparent to the radiation and, on the other hand, it is easy to manipulate (refract) and effectively detect the radiation via its interaction with other atoms and molecules. Neither radio waves, nor x-rays and gamma rays fulfil these requirements. The fact that we are sensitive to a particular spectrum of the light is more or less random, other species are sensitive to a little bit different spectrum as well as certain human individuals.
Sorry for the lengthy reply 🙂
Hi Denis
Your long reply is appreciated 🙂
You make some good points. And I won’t argue much longer on this point because I was having some fun by raising the idea that you can’t have visible light. At the end of the day it depends how you define “light” and “visible”.
I would define light as that energy in the spectrum in the range 360-780nm. Why that range? Because that is the range we (humans) are sensitive to.
I agree that it would be possible to have some light at, say 550nm, that was not visible because – as in your example – it was a single photon at 550nm. But that’s a separate argument. When people use the phrase “visible light” they are not using it to say explicitly that we can see it; rather, they are using the word “light” to represent the electromagnetic spectrum and then the word “visible” to specify which part of light they are referring to (that is, the range 360-780nm). My point is simply that they don’t need to further specify light with the word visible because by saying light they are already restricting themselves to the range of radiation to which we are sensitive. Do you get my point? So I have no issue at all with the phrase “visible spectrum”. That is absolutely fine. But I don’t like “visible light”. By saying visible light it is as if people think the whole electromagnetic spectrum is light and that we are only sensitive to a narrow range of wavelengths of light.
Do you define the word light as being the whole electromagnetic spectrum? Perhaps you do.
I just did a search for some definitions of the word light and here they are:
– the medium of illumination that makes sight possible
– the sensation experienced when electromagnetic radiation within the visible spectrum falls on the retina of the eye
– the natural agent that stimulates sight and makes things visible
– the brightness that comes from the sun, fire, etc. and from electrical devices, and that allows things to be seen
Now, I don’t necessarily agree with all of these definitions. But they all would point to the idea that light is the visible spectrum. That is, that X-rays, gamma rays, radio waves etc are not light. So the term light already specifies that we are talking about radiation in the range to which we are sensitive (360-780nm) and so the adjective visible in the phrase “visible light” is redundant. Whether we can actually see light depends on many things as you say (such as whether we are trichromats or dichromats and whether there is more than one phone etc) but that is a separate matter.
Dear Stephen,
I enjoy the discussion with you so I would like to continue a little bit more.
My point is that there is a very natural definition of light. It is literally defined by Nature via constants of fundamental interactions.
First of all, light is not the whole electromagnetic spectrum. As I mentioned above light can be defined as radiation which behaves in any physical experiments similar to the radiation of visible spectrum. This definition limits the spectral range for light. It is limited from above by the fact that light wavelength should be longer than the size of atoms. The radiation with wavelength comparable or shorter than atomic size behaves substantially different from the rest of the electromagnetic spectrum. Physics changes at this wavelength scale. For example this radiation (we call it X-rays) can ionise atoms (it breaks atoms). You can’t make lenses for X-rays in a usual refractive way. So it is really like a new different world starts at this wavelength scale defined by the size of atoms which is defined by fundamental constants of nature.
On the other hand, wavelength of light should not be too big or frequency should not be too low. Because the strength of radiation coupling to atoms goes down as frequency^3 (if I remember correctly). So coupling to atoms of radiation below infrared light is too week in free space and physics changes substantially again (almost everything is transparent for radio wave and micro waves).
Coupling of radiation to atoms is so important because we consist of atoms. It means we can efficiently interact only with certain electromagnetic spectrum (from infrared to ultraviolet) which is convent to call “light”. Just by chance it happened so that we interact better with a radiation in range of 360-780nm. If our sun would be different we could easily become more infrared or UV sensitive. Furthermore, if at some point we will move to live on a planet at some other star we will adapt to see another spectrum of light.
This “physical” definition of light is free from any subjective feeling what is visible and what is not. It will even “survive” our migration to other stars :). It can only fail in some other Universe with different electromagnetic coupling and different atoms of size of an apple or a watermelon.
P.S. Apparently this guy sees something at 340nm which I guess should be called “invisible light” according to your definition of light 🙂
Hi Denis
I’m struggling a bit to see where we agree and disagree. We both agree that light is only part of the electromagnetic spectrum.
I think that the approximate wavelength range 360-780nm. I would define light as the energy 360-780nm. However, are you saying that your definition of light is broader than that? If you are saying, for example, that light is, say, 300-800nm then I would accept that there is visible light (360-780nm) and invisible light (300-360nm and 780-800nm). I wonder if you could confirm if I have your position correct? In short, what wavelength range do you ascribe to light?
Steve