Painting the Sky, The Science Behind the Formation of Rainbows

Painting the Sky, The Science Behind the Formation of Rainbows

Rainbows are one of nature's most enchanting and universally recognized optical phenomena. These colourful arcs across the sky have inspired myths, art, and scientific inquiry for centuries. In this blog post, we'll dive into the captivating science behind the formation of rainbows.

The Basics of Rainbows

Prism of Light: Rainbows are created when sunlight is refracted (bent) and internally reflected inside raindrops in the atmosphere. This bending and reflection of light transform it into a spectrum of colours.

Dispersion: When light enters a raindrop, it slows down and bends due to the change in speed as it moves from air into denser water. This bending, or refraction, is different for each colour of light because each colour has a different wavelength. This phenomenon is known as dispersion.

Internal Reflection: Once inside the raindrop, the light undergoes internal reflection off the inner surface of the raindrop. The light reflects off the inside surface, bounces around, and exits the drop at various angles.

The Formation Process

Entry Angle: The angle at which sunlight enters the raindrop is crucial. To form a rainbow, the sunlight must enter the raindrop at an angle between 40 and 42 degrees from the direction opposite to the sunlight.

Dispersion Inside the Raindrop: As the sunlight undergoes dispersion and internal reflection, it separates into its component colours, with red being bent the least and violet the most.

Multiple Internal Reflections: The light undergoes multiple internal reflections as it bounces around inside the raindrop. This process reinforces the separation of colours.

Exit Angle: Eventually, the light exits the raindrop, typically at angles ranging from 138 to 142 degrees from the direction opposite to the sunlight.

Formation of the Primary Rainbow: The light exits the raindrop as a circular arc of colours, forming the primary rainbow. This is the familiar seven-colour rainbow with red on the outer edge and violet on the inner edge.

Secondary Rainbows

Double Reflection: In addition to the primary rainbow, a secondary rainbow can sometimes be observed. It is formed by an additional internal reflection inside the raindrop.

Reversed Colors: The secondary rainbow has its colours reversed, with violet on the outer edge and red on the inner edge. It is dimmer than the primary rainbow due to the extra reflection and dispersion.

Supernumerary Rainbows

Additional Bands: Under certain conditions, supernumerary rainbows can appear as faint, closely spaced bands of light inside the primary rainbow. These bands result from interference between waves of light.

Rainbows are not only a marvel of nature but also a testament to the fundamental principles of optics. They reveal how sunlight, a seemingly uniform white light, can be separated into its constituent colours through the processes of refraction, dispersion, and internal reflection inside raindrops. While rainbows are a common meteorological occurrence, they never fail to captivate our imagination and remind us of the intricate beauty hidden within the natural world.