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# Ok this left me scratching my head?

##### New member
:wtf: How do you find the colour of light from the frequency??
That's what I've been trying to tell you. (Wavelength) * (frequency) = speed, since speed is a constant in air, wavelength and frequency are inextricably linked. The speed of light in vacuum/air is ~3(10)^8 m/s, always, for all wavelengths. So speed divided by wavelength is frequency. Frequency is exactly as good as wavelength, because we're operating in a regime where n=1. Since n=1, and the speed of light is constant, wavelength and frequency are inextricably linked. Frequency is just as good at indicating color as wavelength is.

#### Moptsp

##### New member
I guess because frequency is inversely related.
Code:
``````// c is speed of light in vaccum
// w is wavelength
// f is frequency

c = 29,979,245,800 cm/s
w = 532 nm

f = c/w

// get frequency of 532nm at a time frame of 1 second at a measurement of 1 cm, in Hertz.
f = (29,979,245,800 cm/s)/(532 nm)
= f = 56351965.789474
= f = 5.6352 Hz

// This is obviously reversible.
5.6352 Hz = (29,979,245,800 cm/s)/(532 nm)

// If that's true, then:
56351965.789474 = (29,979,245,800 cm/s)/w
= w 56351965.789474 = (29,979,245,800 cm/s)
= w = (29,979,245,800 cm/s) / 56351965.789474
= w = 531.99999. . .
= w = rounded 532``````
That is in a vacuum though.
-Moptsp

#### maxh

##### New member
BTW, it may actually be better or at least more accurate to relate color to frequency instead of wavelength, because the frequency doesn't really change. The speed of light changes as it enters different mediums, thereby causing the wavelength to change. If you shine your green laser into a glass of water then out the other side, you wouldn't say it's green before it hits the glass/water, then some other color, then green again when it exits, would you? The wavelength just changed, though.

Imagine sound waves. a 1000 hz note passing through air has a wavelength of ~13.5 inches. In water its wavelength will be ~3.1 inches. But the frequency is still 1000 hz and it still has the same pitch, even if you listen to it underwater. Good thing they refer to the pitch of sound by its frequency (hz) rather than its wavelength!

But then you start thinking about red shift and other stuff, and maybe it'd be best to relate a photon's color to its energy...

#### bobhaha

##### New member
That's what I've been trying to tell you. (Wavelength) * (frequency) = speed, since speed is a constant in air, wavelength and frequency are inextricably linked. The speed of light in vacuum/air is ~3(10)^8 m/s, always, for all wavelengths. So speed divided by wavelength is frequency. Frequency is exactly as good as wavelength, because we're operating in a regime where n=1. Since n=1, and the speed of light is constant, wavelength and frequency are inextricably linked. Frequency is just as good at indicating color as wavelength is.
omg ok... it just clicked! I get it. But you don't understand what I'm saying... with the frequency, yes you can FIND the colour, but I'm saying wavelength directly describes the colour, eg 405nm (violet), 650 (red) ect...

BTW, it may actually be better or at least more accurate to relate color to frequency instead of wavelength, because the frequency doesn't really change. The speed of light changes as it enters different mediums, thereby causing the wavelength to change. If you shine your green laser into a glass of water then out the other side, you wouldn't say it's green before it hits the glass/water, then some other color, then green again when it exits, would you? The wavelength just changed, though.

Imagine sound waves. a 1000 hz note passing through air has a wavelength of ~13.5 inches. In water its wavelength will be ~3.1 inches. But the frequency is still 1000 hz and it still has the same pitch, even if you listen to it underwater. Good thing they refer to the pitch of sound by its frequency (hz) rather than its wavelength!

But then you start thinking about red shift and other stuff, and maybe it'd be best to relate a photon's color to its energy...
Yes thats correct

Take this picture for example

As the wave hits a "slower" medium, the wavelength would slow, frequency stay the same. But this is not what we are talking about! I'm saying that the colour of light is directly related to its wavelength. Yes you can find the colour of the light with frequency, but isn't that finding the wavelength anyway? refer to the image, if you find the frequency before and after the medium change, im pretty sure you can figure out the wavelength... IIRC its snells law. With that wavelength you know what colour it is... right??

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#### Moptsp

##### New member
BTW, it may actually be better or at least more accurate to relate color to frequency instead of wavelength, because the frequency doesn't really change. The speed of light changes as it enters different mediums, thereby causing the wavelength to change. If you shine your green laser into a glass of water then out the other side, you wouldn't say it's green before it hits the glass/water, then some other color, then green again when it exits, would you? The wavelength just changed, though.

Imagine sound waves. a 1000 hz note passing through air has a wavelength of ~13.5 inches. In water its wavelength will be ~3.1 inches. But the frequency is still 1000 hz and it still has the same pitch, even if you listen to it underwater. Good thing they refer to the pitch of sound by its frequency (hz) rather than its wavelength!

But then you start thinking about red shift and other stuff, and maybe it'd be best to relate a photon's color to its energy...
It sounds like what your saying is that what color we perceive is related to all three, the speed of light, the wavelength, and the frequency.

Because it would look different if the wavelength changed and the frequency stayed the same. But then again that's not possible. That's why I said three things, the speed of light. It slows down to allow what you said, the change between the wavelength and frequency.

If this is true, frequency is still inversely related too wavelength. Because speed changed to adjust it/make it possible.

I'm no expert at this as I haven't studied it too much, so I may be wrong.
-Moptsp

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#### c4r0

##### New member
You know what guys? I think that the 'color' itself has not much to do with physics at all.
1. For example yellow color can be a monochromatic wave with ~590nm wavelength (in vacuum/air to be precise ), or 650nm mixed with 532nm with appropriate proportions - both are the same yellow color.
2. People see colors slightly different. For example, my brother even notices the different recognition of color in his left and right eye...

So IMO color is only a subjective measure of light properties determined by a kind of detector which a human eye is. It's corelated with frequency, photon energy and also wavelength (because the speed of light INSIDE the eye, in point where it hits the 'detectors' is always the same ), but can't be used for precise description of the light.

And if we're talking about physics, I think that the thing describing an EM wave is frequency (that is ALWAYS propotional to the photon energy), because the wavelength depends on the propagation medium, but you've already said it.

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#### Moptsp

##### New member
You know what guys? I think that the 'color' itself has not much to do with physics at all.
1. For example yellow color can be a monochromatic wave with ~590nm wavelength (in vacuum/air to be precise ), or 650nm mixed with 532nm with appropriate proportions - both are the same yellow color.
2. People see colors slightly different. For example, my brother even notices the different recognition of color in his left and right eye...

So IMO color is only a subjective measure of light properties determined by a kind of detector which a human eye is. It's corelated with frequency, photon energy and also wavelength (because the speed of light INSIDE the eye, in point where it hits the 'detectors' is always the same ), but can't be used for precise description of the light.

And if we're talking about physics, I think that the thing describing an EM wave is frequency (that is ALWAYS propotional to the photon energy), because the wavelength depends on the propagation medium, but you've already said it.
Yeah you sound correct. Though all three (wavelength, frequency, speed) affect how we see it.

I was thinking last night and it hit me (like duh) that you can't define a wavelength as a color, because 1(what you said), and two, you still have two unknowns. At least in this simple formula (if it's correct):
Code:
``````// w = wavelength, f = frequency, s = speed(light) where the speed equals wavelength times the frequency.

// get the speed
s = w * f

// get the wavelength
w = s / f

// get the frequency
f = s / w``````
So you can easily see that a color cannot be defined as something unless you have at least one other unknown.

When someone says 532nm, and define it as green they are using a constant s in a vacuum (~29,979,245,800 cm/s).
Code:
``````f * w = s

// without the constant, invalid
f * 532 = 0
f * 532 - 0 = 0 // invalid

// with the constant, valid
f * 532 = (~29,979,245,800 cm/s)
f * 532 - (~29,979,245,800 cm/s) = 0 // valid``````
So you can only define something as something if you have all values. (only need two, you can solve for the third).

You can even define 5.6352 Hz as green if you gave either s or w. Or even (~29,979,245,800 cm/s) as green if you have w or f!

-Moptsp

#### bobhaha

##### New member
f * w = s

// without the constant, invalid
f * 532 = 0
f * 532 - 0 = 0 // invalid

// with the constant, valid
f * 532 = (~29,979,245,800 cm/s)
f * 532 - (~29,979,245,800 cm/s) = 0 // valid
There you go! you just proved my point, with frequency you are finding wavelength (btw I looked it up in an old textbook and colour's are measured in "nm" and in rare occasions "THz") and by finding this wavelength you know where on the spectrum it lies, therefore telling you the colour... do you get what I'm saying or am I not making it clear?? :thinking:

After being here for a short while and reading a lot of post I thought I had started to get a handle on how lasers work and what the different terms here were referring to.

I was under the impression that the nm of a laser beam is the mesurment of the wavelength and told you what color the beam would be.

For instance all the green lasers I've been reading about are 532nm.

So in my head when I saw 532nm I think of it as a green laser.

Referring to the OP as i stated already, yes nm does determine the colour. The wavelength refers to a number along the spectrum chart, which exactly corresponds to a colour of that certain wavelength.

EDIT:​

Just did a little research on THz and I admit I'm wrong. Basically this is what I was thinking, all waves have a frequency, with this frequency and the speed of the wave, you find the wavelength, that wavelength then corresponds to a number in the spectrum telling you its colour. I did a bit more research and found that this is also the case for frequency which is actully measured in terahertz (THz). With just the frequency of light you can also find its colour (refer to picture below). This would also explain the doppler effect, which my thoughts could not satisfy.

Nm is still the base unit, but light can also be described in frequency... now this is straying away from the question, but its all good! Question was does nm determine the colour... basic answer is yes, if you want a more complex answer.. read the whole thread LOL!!

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#### Lasher

##### New member
Hey you guys are great! I gave +rep for those well thought out answers. This thread should be renamed and stickied.

Thanks again guys I've learned a lot from this

Lasher

#### Moptsp

##### New member
@Lasher -
I think I pretty much solved it in my post above bobhaha but I didn't get a +rep. :O

@bobhaha -
Yep, I think we all get it now and are all on the same track. lol
-Moptsp

#### bobhaha

##### New member
LOL I know... I learnt something new.. so I wanted to express it, in a way thats more informative, rather than explaining to some one.. if you know what i mean, its kinda hard to read a thread, and know whats going on if two or three people are talking to each other.

BTW +rep for you... tottaly forgot, you really got me thinking, that in turn help me learn something new/broudned my understanding... and i think that is worth a +rep in my books!