Refraction in a prism

[petrushka.googol]

As we all know refraction splits while light into its component colors. Within a prism light is reduced in speed from its normal velocity to a lower velocity. Does this velocity differ for different color components of white light ? (VIBGYOR). If so, is this evident in the output of different frequencies from a prism (colors in white light) ?

[Strange]

Yes, exactly. The different velocity for each frequency causes each colour to be refracted at a slightly different and so they get split up.

[Sensei]

 

I realized this could be checked experimentally.

On bottom we have laser emitting red, green and blue photons "at the same time".

Then they have to pass through medium with exactly the same distance, but slightly different refractive index for each photon.

Then after exiting medium they should be out of original sync, first delayed more than second, than third, if velocity of photons in medium were different.

 

[swansont]

Experiment Split Photons.png

 

I realized this could be checked experimentally.

On bottom we have laser emitting red, green and blue photons "at the same time".

Then they have to pass through medium with exactly the same distance, but slightly different refractive index for each photon.

Then after exiting medium they should be out of original sync, first delayed more than second, than third, if velocity of photons in medium were different.

 

 

Dispersion in optical fibers is the result of this.

[magnocrat]

The prism acts as a filter but is each colour what we call monochromatic light? I have come across the term monochrome and take it to mean of one colour.

[swansont]

The prism acts as a filter but is each colour what we call monochromatic light? I have come across the term monochrome and take it to mean of one colour.

 

It depends on the spatial extent of the light you are sampling. As the angular size goes to zero, the light becomes more monochromatic. ("monochromatic" is not a binary condition)

[Strange]

Although we are used to thinking of the spectrum as separate colours (Newton split the blue end into indigo and violet so there would be 7 colours to match his mystical ideas) there is really a continuum of gradually changing colours as you across the spectrum.

[StringJunky]

Although we are used to thinking of the spectrum as separate colours (Newton split the blue end into indigo and violet so there would be 7 colours to match his mystical ideas) there is really a continuum of gradually changing colours as you across the spectrum.

Citation please

[swansont]

Citation please

 

http://www.universetoday.com/50720/light-spectrum/

 

"To an astronomer, a light spectrum has two main components, the continuum and the lines (sometimes bands as well).

…

And the continuum? Well, it’s the part that isn’t lines! It varies smoothly, and generally slowly, across the spectrum."

 

http://www.astronomynotes.com/light/s4.htm

 

"A rainbow is an example of a continuous spectrum."

[StringJunky]

 

http://www.universetoday.com/50720/light-spectrum/

 

"To an astronomer, a light spectrum has two main components, the continuum and the lines (sometimes bands as well).

…

And the continuum? Well, it’s the part that isn’t lines! It varies smoothly, and generally slowly, across the spectrum."

 

http://www.astronomynotes.com/light/s4.htm

 

"A rainbow is an example of a continuous spectrum."

Thanks. Just focusing on the visible spectrum: Is each colour a a discrete frequency or is there a set of frequencies within each colour? I've just read the physiology of the eye brings about the apparent separation into the rainbow.

Edited January 28, 2015 by StringJunky

[Strange]

Thanks swansont. This will, of course, depend on the source as well. I assume that the continous spectrum from the Sun is (approximates?) a black body.

 

As for how many colours we see, and the frequency range of each, this depends partly on the physiology of the eye, partly on how much care we take distinguishing colours (and how many names for colours you have in your vocabulary), what language you speak, gender (slightly) and possibly other factors.

 

Here is a great article on this: http://blog.xkcd.com/2010/05/03/color-survey-results/

(Warning: strong language!)

[Fuzzwood]

And even monochromatic light is not really monochromatic. It's always a Gaussian centered around the most intense wavelength of the light emitted.

[swansont]

Thanks. Just focusing on the visible spectrum: Is each colour a a discrete frequency or is there a set of frequencies within each colour? I've just read the physiology of the eye brings about the apparent separation into the rainbow.

The color separation of a rainbow is spatial, so while the eye plays a role, there is a very real component to the spectrum. As I implied earlier, "monochromatic" is not a simple binary condition. Lasers are considered monochromatic but the have a small range of wavelengths/frequencies, not a delta function. LEDs are single color and might be considered monochromatic, but they can cover a range of tens of nm.

 

"Is each colour a a discrete frequency" requires a clarification. Red might be considered to be all visible wavelengths from 600nm and up, or you could break it down to smaller ranges, if you wanted to start creating/naming different bands. These can essentially be made arbitrarily small.

[StringJunky]

The color separation of a rainbow is spatial, so while the eye plays a role, there is a very real component to the spectrum. As I implied earlier, "monochromatic" is not a simple binary condition. Lasers are considered monochromatic but the have a small range of wavelengths/frequencies, not a delta function. LEDs are single color and might be considered monochromatic, but they can cover a range of tens of nm.

 

"Is each colour a a discrete frequency" requires a clarification. Red might be considered to be all visible wavelengths from 600nm and up, or you could break it down to smaller ranges, if you wanted to start creating/naming different bands. These can essentially be made arbitrarily small.

Thanks, that's what I wanted to know.

...

Here is a great article on this: http://blog.xkcd.com/2010/05/03/color-survey-results/

(Warning: strong language!)

Funny.

[John Cuthber]

And even monochromatic light is not really monochromatic. It's always a Gaussian centered around the most intense wavelength of the light emitted.

It's not always Gaussian.

http://en.wikipedia.org/wiki/Spectral_line#Pressure_broadening

.

[magnocrat]

Is there something special about what we call light or do we see it as special because we have eyes. I'm told some animals see colour differently though its hard to believe we can get inside an animals brain to know this is true.

[swansont]

Is there something special about what we call light or do we see it as special because we have eyes. I'm told some animals see colour differently though its hard to believe we can get inside an animals brain to know this is true.

 

We have receptors that span a certain wavelength range. Other animals span a different range. I don't imagine it's overly hard to test for the wavelength sensitivity of those receptors, or just see if a critter reacts to a light that has a narrow spectral output outside of the normal ~400-700nm range we consider to be visible light.

[Strange]

To add to that, we have three differnt (overlapping) receptors. Some birds have four, for example, and some of these extend into the ultra-violet. As such, it seems clear that they would see things that we can't (and, perhaps, vice versa).

 

But we certainly can't get inside their heads to know what the experience is like. But then we can't do that with other humans either (see also: qualia).

 

One of the articles I read on this subject included demonstrations of creating "impossible" colours such as yellow-blue. It's a weird experience to see somethign which is not quite blue and not quite yellow!

[magnocrat]

It may seem a silly question but why do our receptors respond to those particular colours? Is it because the sun throws out more of those so natural selection picked the most prolific source? Its true that the sun looks yellow so maybe our eyes are yellow orentated.

[swansont]

It may seem a silly question but why do our receptors respond to those particular colours? Is it because the sun throws out more of those so natural selection picked the most prolific source?

 

Not a lot of evolutionary sense in developing receptors for a range where there is little or no signal.

[Sensei]

 

Not a lot of evolutionary sense in developing receptors for a range where there is little or no signal.

 

Or reverse - flood of data like in microwave and IR region of spectrum. Which is visible in IR camera.

[Strange]

 

Not a lot of evolutionary sense in developing receptors for a range where there is little or no signal.

 

Plus what is biologically feasible in terms of lenses and sensor pigments.

Its true that the sun looks yellow so maybe our eyes are yellow orentated.

 

A lot of people would argue with that! (It can look yellow, e.g. at sunset)

[StringJunky]

It may seem a silly question but why do our receptors respond to those particular colours? Is it because the sun throws out more of those so natural selection picked the most prolific source? Its true that the sun looks yellow so maybe our eyes are yellow orentated.

Sensitivity to a particular range in the spectrum evolved because that range is what aided us in our reproductive success. If a trait improves reproductive potential it will most likely persist over traits that don't.

[Sensei]

A lot of people would argue with that! (It can look yellow, e.g. at sunset)

 

If we cut blue photons (f.e. reflections, filter) from white spectrum, there will remain red and green,

Red and green photons are read by our brain as yellow.

 

I made this:

(kinda yellow on my 1920x1080 pixels monitor)

 

Zoom in:

 

Just red and green pixels in columns.

 

I mixed them in odd rows, blue filtered just 50%:

 

Edited January 30, 2015 by Sensei

[magnocrat]

These remind me at school ( long ago now) we were shown a wheel of rainbow colours and when it was spun it looked a grayish whiteproving white light consisted of all the rainbow colours. If we mixed fine sands of rainbow colours we would have a sort of whitish sand. How can we be sure when we see a colour it is a pure colour?

White is really a multicolour.