Class 10 > Unit # 12: Electromagnetic Spectrum > Dispersion of Light & Rainbow

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Optics - Dispersion of Light, Spectrum, and Rainbows

Define Dispersion of light and explain the spectrum produced by white light.

Dispersion of Light

"White light is not a single color but a mixture of all the spectrum colors. The splitting of white light into its constituent colors when it passes through a refracting medium like a glass prism is called dispersion of white light."

A glass prism refracts each individual color differently because the refractive index of glass varies slightly with the wavelength of light.

Figure: Refraction and dispersion of white light into component colors through a glass prism.

The Spectrum of White Light

The color pattern produced by the dispersion of light is called a spectrum. A prism refracts light at both of its refracting surfaces, spreading out the range of colors.

When a narrow beam of white light splits, the distinct color sequence produced follows the standard acronym:

V - I - B - G - Y - O - R

Standing for: Violet, Indigo, Blue, Green, Yellow, Orange, and Red.

Speed, Wavelength, and Deviation

The speed and angular deviation of each color component vary depending on its wavelength:

• Red Light: Possesses the longest wavelength, travels with the maximum speed inside the glass prism, and experiences the slightest deviation.
• Violet Light: Possesses the shortest wavelength, travels with the minimum speed inside the glass prism, and experiences the most deviation.

How are rainbows formed?

Rainbows

"A rainbow is an optical phenomenon that serves as an excellent demonstration of light dispersion, proving that visible sunlight consists of a spectrum of distinct wavelengths."

Figure: Mechanics of refraction, internal reflection, and dispersion inside a suspended water droplet forming a rainbow.

To observe a rainbow in the sky, you must look into an area of the atmosphere filled with suspended droplets of water or a light mist, typically at an angle of approximately 40 degrees above ground level, with your back to the sun.

Optical Mechanics inside Water Droplets

Every individual droplet of water acts as a tiny spherical prism:

1. Refraction on Entry: Sunlight enters a water droplet, refracting (bending toward the normal) and partially dispersing into its component colors at the front surface.
2. Internal Reflection: The light rays travel across the interior of the droplet and undergo total internal reflection off the back inner surface of the droplet.
3. Refraction on Exit: As the light rays travel back to the front surface and exit the droplet into the air, they refract once more (bending away from the normal).

Because different wavelengths refract at slightly different angles during this journey of entry, internal reflection, and exit, the dispersed colors reach the observer's eye at distinct angles, creating the magnificent circular arc of the rainbow.

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