White Light and the rest of spectrum.

I know this may sound a little ridiculous, but i cant seem to understand what white light is. As far as i know light has a certain frequency. For example, the color red has a frequency of 650nm. Blue has a frequency of 450nm. Basically, every color has a signature frequency which it, and only it, has.

So this leads me to the question of what frequency is white light? If white light is the combination of all the colors, does that mean it doesnt have a set frequency. I dont understand how one frequency(white light) breaks up into the different frequencies that are the colors.

I must be missing something very fundamental. Someone please clear this up for me.

Thanks

Use math or satistic...

add all the frequencies of the different colors and find the mean or median or average....or something....


I think that white light has frequency...I would the same measuring tool for finding out the Hz of Blue or Red color...

As I understand it, asking what the wavelength (I think were talking about wavelength, not frequency) of white light is, is analogous to asking what tone was used to write the song 'Eleanor Rigby.' Its a combination of frequencies/tones.

[edit on 18-3-2005 by mattison0922]

[edit on 18-3-2005 by mattison0922]

If I remeber correctly, the reason we see black is because all other colors of the spectrum are absorbed, and the opposite for white, all colors are reflected. I might be wrong, but I think it has something to do with that.

Train

that sounds about right.

look at an absorption/emmision spectra and you'll get the same idea

Don't forget...light can be both a wave and a particle, but not exhibited at the same time. When you talk about waves, they can be oriented in an infinite number of positions as opposed to polarized light. White light is the presence of all the wavelengths of light being observed together, thus our brain actually creates the white while our eyes actually see all of the colors. So, it is possibly just a calculation factor in our brains. Light can be very complicated or very simple.

As an artist I was always fascinated by this...

In light, white is ALL color, whereas black is the absence of color.

In pigments though, like paint, white is the absence of color and black is ALL color.

Always thought that was weird....

Man, is this right up my alley or what. I study electromagnetic wave theory for a living. Okay visible light iin proper accordance with em theory goes like this..Red Orange Yellow Green Blue Indigo Violet . Just remember it by ROYGBIV. Anything else is a combination or abscence of. White light is not a fundamental visible color.

Could someone explain some of the wavelengths we dont see, why we dont see them, etc. what does see them (animals, machines)? Are we able to detect all wavelengths, or does our equipment fail at certain points? if someone could answer any of those questions, itd be much appreciated.

EM spectrum e.g. "light" is continuous and the visible spectrum for us meat popsicles is bounded by infrared on the one side and UV at the other. We see light via two mechanisms, reflection and absorbption and addative and/or subtracted wavelengths. Your wall color reflects the actual spectrum of light to give you the color you see, some are absorped some are reflected. Your TV works on the absorption principle, and a trick of the eyeball to take RGB and add it together give you colors. Print color is similar but uses subtractive colors in the CYMK space and again a trick of the eyeball.

Now the nice folks at the CIE have taken the above and given us other definitions of color based on black body radiation. Visible "white" light is actually in the 5000K to 7500K color temperature range. Most define sunlight as white and that is around 6500K. They also define it by a set of coordinates. Then there is something called the CRI (color rendition index) which is a meaure of how accurately various light sources give you a true represenation of color and the word gamma is used to define the actual width of the colorspace that can be displayed in various mediums.

It atcually nifty stuff and you can spend days reading up on this - just google CRI, CIE, SMPTE, Pantone, etc....

One way I use to see the wavelengths on colours is to stick a cut crystal prism in the window and then catch a direct beam of colour in my eye. If you shift slightly to each side you move into another colour. You can see then how many particles each has in comparison to the others and how fast they move. Blue doesn't seem to have as many particles as red for instant and if I remember right its the red side of the spectrum that moves faster than the blue.

White light is actually a lemon colour visually.

"Visible light" is in fact a very narrow range of the EM spectrum which can also be understood as an energy scale. High frequency (low wavelength) radiation has high energy as defined by the equation E = h(nu). h is Planck's constant, equal to 6.626e-34 J * s and nu is the frequency of the wave. This equation can also be re-written in terms of wavelength: E = hc/(lambda) where c is the speed of light, and lambda is the wavelength.

The EM spectrum extends beyond UV on the high energy side to include X-rays, gamma-rays, etc. On the low energy side, it goes down into microwaves, and lower. Any old modern physics text will go through this in a lot more detail.

In quantum mechanics, UV and visible radiation is often (always?) generated by changes to the electronic structure of atoms and molecules. IR radiation is generally associated with the variation of the vibrational frequency of molecules. Microwaves are generally associated with changes to the frequency of molecular rotations. UV and visible light can be absorbed by molecules and internally converted to lower energy light (ie. IR & microwaves).

We see visible light because our eyes are only sensitive to a narrow range of frequencies. You cannot see light unless it is directed towards your eye. For example, the air between a light bulb and an object is filled with the EM wave, but you only see the object that reflects or scatters the light. Light is reflected/scattered off objects and into our eyes. The rods & cones absorb the photons and generate electrical signals that our brain interprets as a 'color'. Other parts of our eyes (the cornea, for example) absorbs some IR frequencies and prevents the photons from reaching the retina. The eye is unbelievably sensitive to light. Some believe that the eye is efficient enough to detect individual photons at some frequencies.

Sorry, that's a lot of rambling. Hope you find it helpful.

A few people have done a splendid job of putting explanations into text, but here's a graphic which may help clarify what's being said.



Mayet stated that red moves faster than blue, which is untrue. I think what was meant is that red has a longer wavelength than blue. The speed of light is constant, for all types.

Also, people who have had surgery for glacouma can see into the ultraviolet part of the spectrum.

Actually I was saying that red moves faster than the blue, you can actually see it when you attempt......successfully the experiment I posted above.

The different colours 'pixels' move at a different speed, or visually appear to. its quite beautiful to experience and doesn't harm the eyes.

a cut prism throws rainbows all around the room, i used a faceted sphere to do it with. follow a strong rainbow and stand in front of it so that it is over your eyes. Move slightly through the colours and you will see it. I am going to do it again in the morning, it has been years since I have done it, to see which colour moves faster, I am pretty sure the blue has the less visual particles, sort of in a pixel by inch way before you move to the next colour and I am not sure whether its the yellow or red that moves faster. I have managed to cross beams to produce other colours such as bright pink and aqua blue in thick bands, that was another fun thing.



[edit on 19-3-2005 by Mayet]

The speed of the light is the same. You're just seeing the oscillations of the light. It moves in waves, so you'll be seeing the crests and troughs. At the bluer end of the spectrum the waves are much closer together than they are at the redder end of the spectrum.

Originally posted by cmdrkeenkid
The speed of the light is the same. You're just seeing the oscillations of the light. It moves in waves, so you'll be seeing the crests and troughs. At the bluer end of the spectrum the waves are much closer together than they are at the redder end of the spectrum.

Thanks for the pictoral post of the EM spectrum - I was too lazy to find one last night.

Frankly, I didn't completely follow the post to which you were responding, so I'm not sure what it was he was trying to describe. What I do know, is that he wasn't observing the actual troughs and crests of light oscillation. Consider a doubled Nd:YAG laser, which is green with a central wavelength of 532 nm. Using c = (nu) * (lambda), the oscillation frequency of green light is 5.63e14 cycles per second. By way of comparison, TV screeens are refreshed at 60 cycles per second. Clearly, what ever effect Mayet was describing, it wasn't the oscillation of light.

So I guess from all this we can simply say;

Color doesn't really exist, it's side-effect of the brain processing electromagnetic radiation of different oscillation frequencies. The interpretation of white is caused by many different wavelengths of light radiation being incident on the same part of the retina in almost equal proportions at the same, or near same, time.

White light can be filtered into its constituent wavelengths as shown by the crystal prism example.

Here's another interesting question: If a wave of light is ~5000 angstroms peak to peak then how can it possibly interact with something like an atom that is only a few angstroms in width and height?

Only 1 in 5000 atoms would receive the photon energy packet, and therefor the maximum amount of reflection (or photon emission) would be at most 5000 time less than the incident light.

Easy man. Please explain photon packets and how it applies to white light. Lets throw out another principle that may help clear things up. The speed of all em energy is the same. Its the frequency that changes and differentiates one from another. Apparantly this means the entire em spectrum travels at the same speed. I believe this was what Einstein was getting at. But many experiments have changed the speed so what now?

The second part of my post doesn't have anything to do with white light. It's about wave or particle interacting with an atom when the atom is ~5000 times smaller than a wavelength of light.

I'm saying that in this scenario, if you consider the light to be a single particle (photon(tiny packet of energy)) then it should only have the chance to interact with 1 in 5000 atoms.

If you look at it the other way, as an electromagnetic wave, the atom is what you would call an electrically small antenna, incapable of interacting with waves of such giganatic size compared to its own dimensions.

[edit on 20-3-2005 by electric]

Originally posted by electricI'm saying that in this scenario, if you consider the light to be a single particle (photon(tiny packet of energy)) then it should only have the chance to interact with 1 in 5000 atoms.

That's correct, if you have a layer that is one atom thick and just one photon running around.



If you look at it the other way, as an electromagnetic wave, the atom is what you would call an electrically small antenna, incapable of interacting with waves of such giganatic size compared to its own dimensions.
Remember, that you're actually dealing with surfaces that are millions of atoms thick and in compounds, the distance between atoms is quite small... so in a compound, the "size" isn't a single atom but rather a chained surface that can be thousands of times larger than a single atom.