# Monochromatic RGB light sources passing through a prism

• KaylaT
In summary, when a white light source is incident on a prism, chromatic dispersion occurs due to the difference in index of refraction for shorter and longer wavelengths. However, when monochromatic light of different wavelengths is incident on a prism, the emergent rays are parallel to each other due to symmetry in the block's entry and exit surfaces. This is in contrast to the dispersion seen with white light.
KaylaT
It is well known that chromatic dispersion occurs when a white light source is incident on a prism. The index of refraction (of the prism) is greater for shorter wavelengths hence the colours emerge from the other side with an angular spread. However I am interested to know the detailed reason why when monochromatic light of different wavelengths (say red, green and blue) is incident at the same time on the prism, the emergent rays are parallel to each other, that is there is no dispersion seen as with the white light source. Why would the refracted angles of these three rays be the same even though they have different wavelengths? I would've thought that they would still emerge at different angles with respect to each other, similar to that with the prism but without the other colours involved. Where's my gap in knowledge?

For a prism, this is usually not the case. For a block where entry and exit surfaces are parallel to each other, it is true, and follows from symmetry.

mfb said:
For a prism, this is usually not the case. For a block where entry and exit surfaces are parallel to each other, it is true, and follows from symmetry.

To expand on what mfb said, anything that disperses white light into a spectrum will also spread your colors into a fan. Your premise is flawed. Mfb was trying to guess where you got this wrong idea. He thinks perhaps you were confused by an illustration where the entrance and exit faces are parallel. Here neither white light nor the three colors are fanned out. The rays emerge parallel as you describe.

## 1. What is a monochromatic RGB light source?

A monochromatic RGB light source refers to a type of light that is made up of red, green, and blue wavelengths only. These three colors are the primary colors of light and can be combined to create a wide range of colors.

## 2. What happens when monochromatic RGB light sources pass through a prism?

When monochromatic RGB light sources pass through a prism, they are refracted or bent due to the different wavelengths of light. This causes the light to split into its component colors and creates a rainbow spectrum.

## 3. Why is a prism used to study monochromatic RGB light sources?

A prism is used to study monochromatic RGB light sources because it allows us to see the different wavelengths of light that make up the source. By separating the colors, we can study their properties and understand how they interact with each other.

## 4. How does the angle of incidence affect monochromatic RGB light sources passing through a prism?

The angle of incidence, which is the angle at which the light enters the prism, affects how much the light is refracted. A larger angle of incidence will result in a larger amount of refraction, leading to a wider separation of the colors.

## 5. Can monochromatic RGB light sources pass through a prism in reverse?

Yes, monochromatic RGB light sources can also pass through a prism in reverse, meaning the light is refracted again as it exits the prism. This process is known as dispersion and is used in devices such as prismatic binoculars and spectrometers.

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