Dispersion and Formation of Light

Key Concepts

• Dispersion of light

• Formation of a rainbow

Introduction: 

White light is a mixture of all the colors (wavelengths) at equal intensities. It is nothing but colorless daylight. The sun and other stars produce white light. Artificially, white LEDs and fluorescent light bulbs produce and emit white light. The light given out by incandescent light bulbs is not white. If this white light is incident on a prism, a certain pattern of light is observed, about which we will be learning in this section. 

Explanation: 

When white light passes through a prism, it splits up into its constituent colors. The phenomenon of splitting white light into its constituent colors is called dispersion of light. Here it is due to the refraction of white light by a prism. 

White light, on dispersion, splits up into 7 colors namely violet, indigo, blue, green, yellow, orange, and red. 

This pattern of colors in the spectrum can be remembered easily by using the acronym VIBGYOR. 

Different colors of light bend through different angles w.r.t. the incident ray as they pass through the prism, i.e., the angles of deviation are different for different colors. 

Red light bends the least whereas, the violet light bends the most. 

This is because red light has the longest wavelength and violet light has the shortest wavelength. 

Even if the wavelengths of different colored lights are different in the prism, their frequency is the same. 

Speed of a wave is given by, v = λν. 

This means that the speed of red light is the highest and the speed of violet light is the lowest in the prism. 

When the spectrum of colors emerging from a prism is passed through another identical prism placed in an inverted position, a beam of white light emerges from it. 

Formation of a rainbow: 

A rainbow is a natural spectrum that appears in the sky after a rain shower. 

Dispersion of sunlight by the tiny water droplets present in the atmosphere forms the rainbow. Here, the water droplets act as small prisms and disperse light. They first refract and then disperse the incident sunlight, which is then reflected internally, and finally refracted again when it comes out of the raindrop. 

A rainbow is formed in the opposite direction to that of the sun. 

Due to the dispersion of light and internal reflection, different colors emerge and reach the observer’s eyes

Questions and answers: 

1. Why don’t we see a dispersion of sunlight when it enters the air? 

Answer: 

We do not see a dispersion of sunlight when it enters the air because the speed of various colors of light in the air is the same. 

Therefore, the air is called a non-dispersive medium. 

2. Arrange the colors of light based on their speed in the prism in ascending order. 

Answer: 

Summary

1. The phenomenon of splitting of white light into its constituent colors is called dispersion of light.

2. White light, on dispersion, splits up into 7 colors namely violet, indigo, blue, green, yellow, orange and red.

3. Different colors of light bend through different angles W.r.t the incident ray as they pass through the prism i.e., the angles of deviation are different for different colors. This results in dispersion of light.

4. Even if the wavelengths of different colored lights are different in the prism, their frequency is the same. The speed of red light is the highest and the speed of violet light is the lowest in the prism.

5. When the spectrum of colors emerged from a prism is passed through another identical prism placed in an inverted position, a beam of white light emerges from it.

6. Formation of a rainbow is caused by dispersion of sun light by tiny water droplets, present in the atmosphere.

7. The water droplets refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop, resulting in a spectrum of colors.