Explain the formation of a rainbow.

A rainbow is a stunning natural phenomenon that occurs when sunlight interacts with water droplets in the atmosphere. This interaction involves several physical processes, including refraction, dispersion, total internal reflection, and refraction once again. Let’s explore how rainbow is formed in a simple and detailed way.

Refraction of Sunlight

The process begins when sunlight enters a water droplet. As it passes from air into the water, the light bends, or refracts. This bending occurs because light travels at different speeds in different mediums. White light, which is composed of multiple colors, splits into its seven constituent colors during this refraction. These colors are: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This separation of light is known as dispersion.

Among these colors, red light bends the least, while violet light bends the most. This difference in the bending of light waves is what leads to the formation of the color spectrum we see in a rainbow.

Total Internal Reflection

Inside the raindrop, after the sunlight is refracted, the light strikes the inner surface of the droplet. At this point, the light is now traveling at an angle that could cause it to reflect back inside the droplet if the angle exceeds a certain threshold known as the critical angle (around 48°). This process is called total internal reflection, which ensures that the light stays inside the droplet, instead of escaping.

Second Refraction

After the light has undergone total internal reflection, it travels to the other side of the droplet. As it exits the droplet, it undergoes a second refraction. This further separates the light into its various colors, contributing to the full spectrum that forms the rainbow.

Emergence of Colors

The different colors emerge from the water droplet at slightly different angles. For example, violet light exits at an angle of approximately 40°, while red light exits at about 42°. This difference in angles causes the colors to spread out, creating the arc of a rainbow. The red color, being on the outer edge, appears at the top of the rainbow, and violet, being on the inner edge, appears at the bottom.

Types of Rainbows

There are two main types of rainbows that can be observed:

Primary Rainbow

The primary rainbow is the brighter, inner arc of the rainbow. It is formed by a single internal reflection of light inside the raindrop. The color sequence in the primary rainbow is red on the outer edge and violet on the inner edge.

Secondary Rainbow

The secondary rainbow appears outside the primary rainbow and is fainter in comparison. It is formed by two internal reflections inside the water droplet. In this case, the color sequence is reversed, with red at the bottom and violet at the top.