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

• lockerman2007

#### 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 ?

What properties of light affect its speed ?
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).

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

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

$$c=\lambda*f$$

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

$$c=\lambda*f$$

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

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.

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?

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.

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?

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

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?

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?
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.
sneez said:
I own a prism but when i look at light source through it , it does not produce colors. why is that?
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.

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.

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

I don't quite understand what is the meaning by "see" a higher refractive index, can you explain it in detail ? thank you.
It is just a simple way of conveying that refractive index varies with wavelength.

Claude.

By the way, i still was not able to repeat the prism experiment. I don't belief I am 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...?

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.