White light thru prism: Why aren't separated colors actually mixed?

In summary, the light coming out of the prism seems to be made of stacked colors of the rainbow, not a mixture of the colors.
  • #1
yosimba2000
206
9
Ok, here's the illustration of the experiment: http://media.web.britannica.com/eb-media/10/7710-050-36C066AC.jpg

But if you trace the light path back from the prism and into the white light, it seems as though the white light is actually made up of STACKED colors of the rainbow, not a MIXTURE of the colors.

So here is white light made of stacked colors: http://i.imgur.com/8PHHmoI.png

Here is white light made of mixed colors (still has colors of rainbow, just not in order): http://i.imgur.com/3n6zQaa.png

And here is the hypothesized mixed white light after coming out of prism: http://i.imgur.com/Azhc3Ge.png

But if different colors refract at different angles, and the same colors refract at the same angles as one another, then that means the last picture would be observed. But since we don't observe that in real life, then white light is actually an ordered stack of colors?
 
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  • #2
yosimba2000 said:
Ok, here's the illustration of the experiment: http://media.web.britannica.com/eb-media/10/7710-050-36C066AC.jpg

But if you trace the light path back from the prism and into the white light, it seems as though the white light is actually made up of STACKED colors of the rainbow, not a MIXTURE of the colors.

So here is white light made of stacked colors: http://i.imgur.com/8PHHmoI.png

Here is white light made of mixed colors (still has colors of rainbow, just not in order): http://i.imgur.com/3n6zQaa.png

And here is the hypothesized mixed white light after coming out of prism: http://i.imgur.com/Azhc3Ge.png

But if different colors refract at different angles, and the same colors refract at the same angles as one another, then that means the last picture would be observed. But since we don't observe that in real life, then white light is actually an ordered stack of colors?
The first image is the closest to real life. The other images are just silly.

Use Google Images to see real pictures of a prism separating white light into the ROYGBIV rainbow of colors.
 
  • #3
Like this image:

http://www.bbc.co.uk/staticarchive/76a64e18ce06770fe8376e1985cca969d600d7e8.jpg
76a64e18ce06770fe8376e1985cca969d600d7e8.jpg
 
  • #4
berkeman said:
The first image is the closest to real life. The other images are just silly.

Use Google Images to see real pictures of a prism separating white light into the ROYGBIV rainbow of colors.

Sure, but the question still has to be answered: is white light a mixture of the rainbow or a stack of the rainbow?
 
  • #5
yosimba2000 said:
Sure, but the question still has to be answered: is white light a mixture of the rainbow or a stack of the rainbow?
Mixture. The stack thing seems quite erroneous. Where did you read that term?
 
  • #6
berkeman said:
Mixture. The stack thing seems quite erroneous. Where did you read that term?

I made it up to describe what I was thinking about :D

Also, I think I know my misunderstanding now. The rays I drew in the last picture, if extended, will result in light separation. I just didn't draw the rays long enough. So, then, I guess this means this light prism experiment doesn't show the rainbow below a certain distance because there won't be enough distance for the light rays to separate?
 
  • #7
Your last picture is incorrect. That is not the order of the colors when they emerge from the prism. Berk's picture shows this immediately.

yosimba2000 said:
But if you trace the light path back from the prism and into the white light, it seems as though the white light is actually made up of STACKED colors of the rainbow, not a MIXTURE of the colors.

How so? If I trace the colors back I see a single beam of white light and no evidence of whatever this "stack" is supposed to be.

Are you actually tracing light back through a real prism, or just doing a thought experiment?
 
  • #8
I think you are being mislead by the original Brittanica picture, which is wrong. They show the beam being split completely at the first face of the prism - as if the beam were as you conjecture - which is wrong.
(Edit: I've looked for some better pictures, but most fall into the same error. I think it is very hard to draw the correct picture, either by hand or with most graphics software.)
Say the original beam were 1mm wide, then the red light would come from the whole 1mm width and continue as a 1mm red beam producing 1mm wide band of red light on the screen.
Equally the blue light would come from the whole 1mm beam width in a 1mm beam all the way to the screen and produce a 1mm wide band of blue light.
Similarly for all the other colours - N.B. not just the nominal 7, but the infinite number of colour /wavelength gradations. Each produces its 1mm band on the screen, overlapping and mixing with the adjacent colours.
This effect can be reduced by making the initial beam narrower, which dims the spectrum or by using lenses to focus a divergent beam from a narrow slit, which keeps it brighter.
This is shown in SchoolPhysics . Their diagram shows only two representative rays at the sides of the beam, but you can see that both red and blue light (and all other colours) come from all parts of the beam.
spectrum.png


Edit: Maybe a better diagram of the same thing at Telecopes in OpenEd
spectrum2.jpg

The "polychromatic beam" is the equivalent of the white beam, but what they are looking at may not be white. (If you try this experiment, it might be interesting to use an LED or flourescent lamp as a "white" source. Their spectra are noticeably different from that of sunlight or incandescent light, even with simple equipment. &BTW, if you want to experiment and haven't got a suitable prism, a CD will do an adequate job as a diffraction grating.)
 
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  • #9
Indeed. It's important to understand that the light being sent into the prism in most pictures is a thin beam or column. Without making the light into a beam or column you cannot see the dispersion. If you have a prism, cutting a thin rectangular slit in a piece of thick paper or cardboard should work just fine.
 
  • #10
Drakkith said:
Your last picture is incorrect. That is not the order of the colors when they emerge from the prism. Berk's picture shows this immediately.
How so? If I trace the colors back I see a single beam of white light and no evidence of whatever this "stack" is supposed to be.

Are you actually tracing light back through a real prism, or just doing a thought experiment?

Just a thought experiment based on the Brittanica picture.
Merlin3189 said:
I think you are being mislead by the original Brittanica picture, which is wrong. They show the beam being split completely at the first face of the prism - as if the beam were as you conjecture - which is wrong.
(Edit: I've looked for some better pictures, but most fall into the same error. I think it is very hard to draw the correct picture, either by hand or with most graphics software.)
Say the original beam were 1mm wide, then the red light would come from the whole 1mm width and continue as a 1mm red beam producing 1mm wide band of red light on the screen.
Equally the blue light would come from the whole 1mm beam width in a 1mm beam all the way to the screen and produce a 1mm wide band of blue light.
Similarly for all the other colours - N.B. not just the nominal 7, but the infinite number of colour /wavelength gradations. Each produces its 1mm band on the screen, overlapping and mixing with the adjacent colours.

I see. So all the colors would be just as wide as the original white beam of light. Thanks everyone!
 
  • #11
yosimba2000 said:
I see. So all the colors would be just as wide as the original white beam of light. Thanks everyone!
Experimental limits will always cause spreading of any particular spectral line. Original beam width, chromatic aberration and diffraction will always give uncertainly of the wavelength measured in a spectrometer or spectroscope.
It is precisely the same thing that happens with frequency measurement of other EM waves (or other types of oscillation). The analyser has a finite bandwidth so it will record the presence of frequencies on either side of the nominal measured frequency.
The original naming of the spectral colours really didn't help either. A total arbitrary exercise, based on Numerology!
 
  • #12
Drakkith said:
Without making the light into a beam or column you cannot see the dispersion.
Except at the edges of the beam.

[added] Oops, I now realize that Drak was assuming a beam in the first place. However, even without a narrow beam, you can still see dispersion effects at the edge of a broad "beam" e.g. when looking through a prism at some object. Or through a lens that is poorly corrected for dispersion, as in a cheap camera.

I have to wear strong eyeglasses. I once had a pair that used glass with a high index of refraction to make the lenses thinner. That particular glass also had a lot of dispersion, so objects toward the edges of my field of view had nice colored fringes.
 
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  • #13
As ever, there are other issues. Making the slit narrower will restrict the amount of light getting through. So the pure spectrum display can get too dim to see!
 

Related to White light thru prism: Why aren't separated colors actually mixed?

1. Why does white light get separated into different colors when passing through a prism?

White light is made up of all the visible wavelengths of light, which our eyes perceive as different colors. When white light enters a prism, it is refracted or bent at different angles depending on the wavelength of the light. This separates the white light into its component colors, creating a rainbow effect.

2. How does the prism separate the colors of white light?

The prism is made of a material that has different refractive indices for different wavelengths of light. This means that each color of light is bent at a slightly different angle as it passes through the prism, causing them to separate and form a spectrum.

3. Why do the colors not mix back together after being separated by the prism?

The colors do not mix back together because they have different wavelengths and are traveling at different speeds. This means that they have different paths through the prism and will not overlap to create white light again.

4. Can other materials besides a prism separate white light into colors?

Yes, other materials with different refractive indices can also separate white light into colors. For example, raindrops in the sky act as tiny prisms to create a rainbow, and a CD or DVD can also separate light into its component colors.

5. How does the separation of colors in white light by a prism relate to the concept of visible light spectrum?

The separation of colors in white light by a prism shows the continuous spectrum of visible light, which ranges from red to violet. Each color corresponds to a specific wavelength of light, and the order of colors in the spectrum is always the same: red, orange, yellow, green, blue, indigo, and violet.

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