Have you ever wondered what makes colors possible? It’s because Sun’s light has different colors. You can see these colors, from which light is composed of a phenomenon called dispersion of light.

Dispersion of light

Light coming from Sun does not have a fixed wavelength, it actually is composed of a series of wavelengths instead. Infrared light, visible light, and ultraviolet light are coming from the Sun all at once. Most of the ultraviolet light is absorbed by the ozone and the light that enters are mostly visible light and infrared.

These two lights are distinct in terms of the energy they carry due to differences in their wavelength. Further, visible light can have seven different colors in it, namely red, orange, yellow, green, blue, indigo and violet, each with a definite frequency and wavelength.

The splitting of ordinary (white) light into these seven different colors is called dispersion of light. But, the sunlight appears to be colorless, how can such a combination of colors be colorless?

We have to go through a simple experiment to prove that light is composed of several colors. Take a glass prism so that the sunlight passes through one of its edges in an inclined position as in the figure.

You can see different colored stripes as in the rainbow on the screen. This is the dispersion of light. It can be caused due to the difference in velocity of different component colors of light when light passes to a medium. Meanwhile, the color combination of red, orange, yellow, green, blue, indigo, and violet gives rise to the white color of sunlight.

As for colors seen on objects, when this white light is cast upon any object, it reflects a particular combination of colors. This reflected color is the one we see. Rose is red because it can reflect red light while absorbing other colors more.

What causes the dispersion of light?

Light is composed of seven different colors and during the refraction of light, the velocities of different colors differ in a medium other than air. This can cause the dispersion of light. So, it occurs because of the difference in wave velocities of different colors of light in a particular medium.

Dispersion during refraction

The coefficient of refraction, the refractive index is the ratio of the velocity of light in a vacuum to the velocity of light in a given media. The velocity of light is almost constant for different colors of light in air or in a vacuum. But, if we consider other transparent mediums like water, glass, diamond, etc. the velocity differs and refraction occurs.

Since different component colors of light have different velocities in a particular medium, the colors have a different coefficients of refraction. This makes some colors bend more while some colors can bend less while light enters from air to glass or water or any other medium. In particular, red light bends less while violet bends the most, and a band of the other five colors appear in between them.

Hence the colors are divided into their group and we can observe them individually. But, laser light and other monochromatic (having single wavelength) light cannot show dispersion due to the presence of only one color of light.

Why do we need a prism rather than a glass slab?

Refraction occurs through glass slabs too but it does not disperse light effectively due to the parallel nature of its surfaces. This geometry of the glass slab forbids the light to disperse by recombining them to form a group and have a group velocity. If the two refracting surfaces are tilted in contrast to the parallel surface of the slab, dispersion can occur. You can cut the surface of the slab and see it for yourself.

Phenomena associated with the dispersion of light

The most common phenomenon due to the dispersion of light is the occurrence of the rainbow. The colors of the rainbow are just the dispersed colors of sunlight. This is why it has seven colors. While different colors of the sky and ocean are not due to dispersion but due to the scattering of light.

Dispersion is observed in many optical instruments like lenses, optical fibers, and waveguides. Actually, it creates problems in lenses called chromatic aberration, the inability of a lens to focus all the colors of light at a single point. This results in images being blurred or having different unusual colors at the edges.

This problem has to be tackled in optical instruments like microscopes and telescopes by the use of an achromatic combination of lenses. The convex and concave lenses of different materials can be used such as the dispersion of light.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.