# Question about the dispersion of white light in a prism.

1. Feb 11, 2007

### lockerman2007

Hi,
I have a question about the dispersion of white light in prism.
Why different color of light have different speed in prism ?
What properties of light affect its speed ?

2. Feb 11, 2007

### lalbatros

The color.
Remember whitelight is not mono-chromatic: it is a mix of different monochromatics lights or "colors".
An a mono-color or mono-chromatic light is characterised by its frequency (or its wavelength in the given medium).

3. Feb 12, 2007

### lockerman2007

hmm, so as different color of light have different speeds, why they do not separate when they travel to the Earth as they already travelled a long long distance from the sun to the Earth ??

4. Feb 12, 2007

### americanforest

Correct me if I am wrong but, don't all electromagnetic waves have the speed c, 300,000 km/s. The speed is the same for all of them. The valid equation is

$$c=\lambda*f$$

5. Feb 12, 2007

### Moridin

Electromagnetic radiation does indeed have the velocity c, which is 299792458 m/s in vacuum. However, http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/cspeed.html#c1

6. Feb 12, 2007

### Claude Bile

Material dispersion is the variation in refractive index with wavelength. Shorter (optical) wavelengths typically "see" a higher refractive index and thus travel slower in a medium such as glass than longer wavelengths.

Claude.

7. Feb 13, 2007

### sneez

Was there an experiment done where the reverse of prism phenomenon? Different wavelength combined in white-light.

On the same note, is light really "white"? Light is colorless as far a physics,no? Is there known mechanism which would make combination of dark colors produce a brighter color than any of the separate color making that up?

8. Feb 13, 2007

### Claude Bile

Not 100% sure what you mean by the reverse of the prism phenomenon, but you can get anomalous (negative) dispersion where shorter wavelengths "see" a lower refractive index than longer wavelengths, this occurs naturally around absorption bands in a material. Meta-materials can also be engineered to have negative dispersion.

How we perceive each colour is more to do with how our brain interprets signals from our eyes, rather than the nature of the light itself.

Claude.

9. Feb 13, 2007

### sneez

What i mean is; Can we take light at the wavelengths of the colors and put it together to produce 'white light'?

We say that colors are 'contained' in white light. But how that can be? Can we combine all the colors to produce white?

10. Feb 13, 2007

### Claude Bile

Of course, the principle of reversability applies in this situation. If you can split white light into colours, the principle of reversability says that we can recombine them back into white light.

We "see" white when all three of our colour receptors (red, green, blue) are stimulated, because white light is made up of many wavelengths (i.e. it has a broad spectrum). All light you see is in fact made of many wavelengths, even laser light. It is just that when we see something that is "red", we are seeing light that is only stimulating our red receptors and not our green and blue receptors. The term "white" in fact, often refers to a broad spectrum, for example white noise is noise with a broad spectrum.

To reiterate, we only "see" white as being distinct from a combination of red, green, blue and everything else because of how our eyes and brain perceive a spectrum that stimulates all of our colour receptors at once.

Claude.

11. Feb 13, 2007

### sneez

I see, thanx.

another one; can we explain the order of colors as they appear from prism or rainbow by other than corrolation with wavelengths? I own a prism but when i look at light source through it , it does not produce colors. why is that?

12. Feb 14, 2007

### Claude Bile

You have to understand that the only parameter we can characterise an electromagnetic wave with is its wavelength. Colour is not a parameter that is pertinent to electromagnetic waves, it is a parameter that relates to how we perceive different wavelengths when we detect them with our eyes.
A few reasons - firstly you might not have enough light. Try a bright beam in a dark room for the best visibility. Secondly, you need to get the beam to hit the prism at a glancing angle, if you have it near incidence, you will not see much colour spread at all.

Claude.

13. Feb 16, 2007

### lockerman2007

I don't quite understand what is the meaning by "see" a higher refractive index, can you explain it in detail ? thank you.

14. Feb 18, 2007

### Claude Bile

It is just a simple way of conveying that refractive index varies with wavelength.

Claude.

15. Feb 19, 2007

### sneez

By the way, i still was not able to repeat the prism experiment. I dont belief im told everything. I used incadesent 70w bulb as source. When viewed through prism through slant angle there are no colors appearing. However, there are colors at the boundary! Its seems that only when there is boundary there seems to be color arrising. Any thoughts...?

16. Feb 19, 2007

### Claude Bile

When the light encounters a boundary, different wavelengths will refract at different angles, because the angle of refraction is dependant on the refractive index of the prism (which we have already established is wavelength-dependant). It therefore comes as no surprise that the colours are most visible near the boundary of the prism.

In fact, jewellers use this phenomenon by introducing many cut facets into a stone to enhance its sparkle.

Claude.