Have you ever wondered what makes colors possible? Its 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 wavelength instead. The 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 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.

Dispersion of light through prism
Image credit: KUllabs.com

You can see different colored stripes as in the rainbow on the screen. This is 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 particular or 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.

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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 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, refractive index is the ratio of the velocity of light in a vacuum to the velocity of light in a give 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 medium 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 coefficient of refraction. This makes some color to 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 band of other five colors appear in between them.

Hence the colors are divided from 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 we need 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, the dispersion can occur. You can cut the surface of the slab and see it for yourself.

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Phenomena associated with the dispersion of light

Most common phenomenon due to the dispersion of light is the occurrence of 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 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 image to be blur or have different unusual colors at the edges.

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

Author

Ashwin Khadka is a Physics graduate from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer. Apart from writing he is also a researcher, with specialization on thin films for electrodes in solar cells.

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