“Colors are light’s suffering and joy.” - Johann Wolfgang von Goethe
While we did touch upon the subject of prisms when we talked about how a diamond’s cut influences its optical properties, in this blog, we’re going to present a more in-depth approach to how prisms interact with light.
In essence, a prism breaks the light that enters it into the colors of the spectrum due to a combination of optical phenomena: refraction and dispersion. This special characteristic of prisms was documented by physicist Sir Isaac Newton in the 17th century.
Newton determined through a series of experiments involving a triangular glass prism that white light is the sum of the seven colors of the visible spectrum—red, orange, yellow, green, blue, indigo and violet. His discovery replaced Aristotle’s theory of color, which suggested that all the colors result from the combination of white and black.
Newton’s findings were published in his work Optiks and represent an important milestone in the study of color in nature.
CREATING COLOR WITH PRISMS
Compared to how structural colors are formed in the wings of the Morpho butterfly, prisms create color through the interplay of optical phenomena.
When light travels from one transparent medium to another—from air to the glass of a prism, for example—it causes the ray of light to slow down and bend, changing its propagation direction. This phenomenon is called refraction.
Simply put, refraction happens because the air has a different index of refraction than glass, meaning that light travels faster in the air than inside a glass prism since it’s a less dense medium.
After light enters the prism and gets refracted, it’s split into its component wavelengths or colors. This phenomenon is known as dispersion. The reason why chromatic dispersion happens is that the refractive index of the medium varies for each wavelength, thus making them bend at different angles.
To properly understand how a prism refracts and disperses white light forming the visible spectrum, it’s essential to briefly explain light’s dual nature.
In simple terms, light is made of particles (photons) with various wavelengths and frequencies. Short wavelengths have a high frequency (energy) and move faster, while longer wavelengths have a lower frequency and travel more slowly.
Image Source: Wikimedia Commons
Moreover, for each wavelength of light, there’s a different degree of refraction. The two elements are inversely proportional: the shorter the wavelength of light, the higher the amount of refraction.
Therefore, short wavelengths such as violet and blue are slowed down more and undergo more bending than longer wavelengths like red and orange.
Generally, the dispersion of white light is highly accentuated when it exits the prism and a second refraction occurs. That’s why we can clearly see the rainbow reflected on the other side of the prism.
FINDING PRISMS IN NATURE
A perfectly cut diamond has the optical qualities of a prism. Thus, the light entering the diamond and getting refracted and dispersed contributes to the vibrant flashes of color we see when tilting the gemstone.
However, the diamond is just one example of natural prisms. The captivating appearance of a rainbow is also the effect of color dispersion. In this case, the suspended raindrops in the air start behaving like miniature liquid prisms.
When a beam of light hits the spherical surface of a water droplet, it first refracts as it enters the droplet. After the light reflects off the back of the raindrop, it undergoes refraction again as it exists.
During this process, white light is dispersed into the visible spectrum, the most intensity of light being found between 40° and 42°. Depending on the index of refraction of water for each wavelength, a different color will reach back to our eyes.
As we explained in the previous section, since the violet light is refracted most, it emerges on top. Also, the red light is refracted less, which means we can observe it at the top of the rainbow.
Image Source: Encyclopædia Britannica
Another thing worth mentioning is that, in theory, all rainbows are full circles. However, from Earth, we can only see the rainbow as a semicircle, as our eyes can perceive only the wavelengths of light reflected above the horizon. The only way to see rainbows as a whole circle is from an airplane.
Sometimes, we might be able to spot a secondary rainbow in the sky. A double rainbow forms as the result of light reflecting twice within the raindrops. This type of rainbow appears at about 10° on the outer side of the primary rainbow, representing the reflection of the primary arc. That’s why its color order is reversed.
In the eastern culture, double rainbows are considered a symbol of transformation and growth, bringing good fortune to the observers. Also, the primary rainbow is often associated with the material world, while the secondary arc represents the spiritual world.
This article is part of our Connect with Nature blog series, where we explore how colors are created in the natural world while focusing on the relationship between light and color.
We are constantly inspired by nature’s remarkable design and engineering capabilities when it comes to our work. That’s why we strive to create pieces that elegantly capture the sense of wonder that arises through our connection with nature.
Encyclopædia Britannica - Light
Encyclopædia Britannica - Color
Sciencing - What causes the dispersion of white light?