6.3. Cause of Dispersion

Light of different colors (wavelengths) travels at different speeds in a medium like glass.

When light passes through a medium such as glass, each color of light does not travel at the same speed. This is because different wavelengths of light, which correspond to different colors, interact with the material of the glass differently. For example, violet light (the color with the shortest wavelength) travels slower through the glass compared to red light (the color with the longest wavelength). This variation in speed is fundamental to understanding dispersion.

Think of it like a group of people running through a crowded room. The shorter people (representing violet light) may struggle more to navigate through the crowd than the taller people (representing red light). As a result, the shorter people might take longer to get to the other side, just like violet light takes longer to travel through glass.

Due to this, the refractive index of the medium is different for each color.

The refractive index of a medium is a measure of how much the speed of light is reduced inside that medium compared to its speed in a vacuum. Since different colors of light travel at different speeds in glass, they also have different refractive indices. Violet light has a higher refractive index than red light, meaning it bends more when entering or exiting the prism, while red light bends less.

Imagine a group of friends walking through a hallway where the floor is slippery in some places and rough in others. The friends that have easy shoes to run in (like red light) can move quickly through the rough parts, while the ones in heavy boots (like violet light) struggle. This difference in ease of movement represents how different refractive indices affect the bending of light.

Violet light bends the most and red bends the least.

As light passes through a prism, each color is bent by a different amount due to the differences in their refractive indices. Violet light, which travels the slowest in glass, is bent the most as it enters and exits the prism, while red light, which travels the fastest, is bent the least. This bending is what creates the visible separation of colors called dispersion.

Consider how a rainbow forms after rain: it’s as if light enters the prism of water droplets, bending and spreading out into a fan of colors. Just like this, when light passes through a prism, violet, being the most bent, will appear at one end, and red, being the least bent, will showcase at the opposite end.

This separation of colors is the cause of dispersion.

The process where the different colors of light are separated due to their varying speeds and bending angles is known as dispersion. Essentially, because of the changes in speed and refractive direction of each color as they pass through the prism, we can see a spectrum instead of just a white light. This phenomenon is responsible for creating the beautiful range of colors we can observe.

Imagine having a set of crayons where each crayon represents a color of light. If you were to spill water on them, it might mix them up. But if you use a tool like a ruler or a stick to separate them based on size, you see each crayon clearly again. Just like this, a prism acts as a separator that allows us to see individual colors instead of just white light.

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