EBME Forums Forums Free discussion Free discussion Definition of Light

I have always regarded light as having two classes, visible light and invisible light. Visible light is light in the range approximately 400 to 700 nm. Invisible light is all other electromagnetic radiation from gamma rays to radio waves.

A recent discussion in our lab recently revealed that there is a differing viewpoint on this and the other viewpoint suggests that light should be defined as part of the electromagnetic spectrum with wavelenght 400 to 700nm ie light that is visible to the human eye. Gamma rays are defined not as light but as part of the electromagenetic spectrum with wavelength of 0.03 to 0.003nm

So how do you define light, is it the entire electromagnetic spectrum or is it just the visible part?

960 nm infrared used in pulse oximetry is not visable but none the less light. Same for ultra-violet. Heres a website that defines light :
http://fuse.pha.jhu.edu/~wpb/spectroscopy/em_spec.html
but I'm sure other definitions are out there.

I once had a debate with a colleague about Gamma and X ray radiation. He insisted they were the same thing. Are they?

Gamma radiation and X-ray frequencies do overlap, with the lower energy Gamma radiation overlapping with the higher energy X-rays.

It is the origin of the radiation which distinguishes which type it is.

Gamma radiation is usually associated with processes involving the release of energy from the nucleus of the atom, whereas X-rays are usually the result of the manipulation of energetic electrons.

That's my understanding of it, anyway!

To get back to the original question:

Light, to the average Joe Public, could probably be defined at the visible part of the elecromagnetic spectrum, but when you start to look at light-speed and doppler shifts with regard to cosmology, this definition is insufficient.

I would, therefore, define light as an electromagnetic wave having a frequency and wavelength which, when multiplied together always give the same result, namely 300 million metres per second.

BSM,

Actually I think Gamma radiation is associated with shorter characteristic wavelengths hence a higher frequency (classical theory) thus higher energy photons (quantum theory - E=hf). Is this not the case? There is also the aspect of how different energy photons interact with matter i.e. how the energy is deposited in the medium.

Hence Speed of light = wavelength x frequency?

I like this definition:

In my opinion, a light ray is the part of the electromagnetic spectrum which can be seen by the eye.
You get a light ray as you do a gamma ray, X ray, heat ray, UV ray etc.
Light is used to define a certain frequency range or wavelength range of the electromagnetic spectrum, much in the same way that X ray, Gamma ray, Radio wave etc are used to define other frequency/wavelength ranges of the electromagnetic spectrum.
The electromagnetic spectrum has behavioural aspects of both a wave and a particle, the lower the frequency the more like a wave it behaves (Long wave radio is great at bending round buildings etc much better than shortwave), the higher, then the more like a particle it behaves (Gamma rays are destructive). Perhaps this is why radio waves are referred to as a wave rather than a ray (Just a wee thought I had)
Going by that I would say that light just refers to a part of the electromagnetic spectrum ie in the wavelenghth range of 400 to 700 nanometres in the same way that X rays, Gamma rays etc are other parts of the electromagnetic spectrum.
Some poeple generalise and say that X rays etc are another form of light, it is light with a different frequency however you don't hear many poeple saying that light is another form of Gamma Ray.
This is more true of parts of the electromagnetic spectrum with a frequency very close to that of light, ie Infra red and Ultra violet, they are reffered to as forms of light far more than Gamma rays ie Ultra violet light.
The common thing among them is that they are all parts of the electromagnetic spectrum. I went online and typed; define light. Not all but most definitions said "that can be seen by the human eye or by animals".

That's a really good way to put it.

Quote: I would, therefore, define light as an electromagnetic wave having a frequency and wavelength which, when multiplied together always give the same result, namely 300 million metres per second.


My cock-up! Sorry