Seaside Blues: Waves of Water and Light

If we met when I was in the fifth grade and engaged in age-appropriate small talk, you might ask me which colour was my favourite. Without so much as a second thought, I would tell you that it was blue. Why? Because I was in the 'blue house' at school! It was the age of unflinching loyalty to whatever colour plastic badge was pinned to your shirt. Ask me today about my favourite colour, and you'll get the same answer. Though a few decades' worth of staring at shades of blue around me have profoundly influenced why I like this colour so much. My curiosities with the colour blue are now inseparably dissolved in my fascination with the behaviour of light. In what I'm hoping is the first in a series of love letters to this colour, I write about light as it journeys into the Andaman Sea and paints the water in every mood of blue I've ever felt.

At times, I'm on a beach watching the tide recede, and the intertidal zone becomes a narrow spectrum of bright optimism ranging from turquoise through azure to aquamarine. Sometimes, I'm on a SCUBA dive 30 metres underwater, suspended in ink blue while watching barracudas above me cut circles into marine blue. There's also the emotionally packed exit on a ferry that begins with sea blue in the afternoon and descends into the darker depths of an Oxford blue by dusk. How does the sea express so many shades of blue?

One of the first modern explanations came from the late great Lord Rayleigh, who famously explained why the sky was blue. His work showed that light from the sun 'scatters' as it hits gas molecules and microscopic particles in our atmosphere (Rayleigh scattering). Why blue when the sun is sending us a whole bunch of light across the colour spectrum? Blue has the shortest wavelength among the colours visible to us, and that makes it highly prone to scattering. The intensity of scattering, he showed, is inversely proportional to the fourth power of the wavelength. The shorter the wavelength, the more the scattering. This is why our sky is blue!

I ∝ 1/λ^4

The formula for Rayleigh scattering, where I is the intensity of scattering and λ is the wavelength of the beam.

But what did Lord Rayleigh have to say about the sea?

"In the ocean the depth is, of course, adequate to develop the colour, but if the water is clear, there is often nothing to send the light back to the observer. Under these circumstances, the proper colour cannot be seen. The much-admired dark blue of the deep sea has nothing to do with the colour of water, but is simply the blue of the sky seen by reflection.”

- Lord Rayleigh

A leader of scientific inquiry at the time, Rayleigh suggested the ocean (or sea) merely reflected the blue of the sky! Not entirely satisfied with this explanation was a scientist from India, Sir C V Raman. During his travels across the Mediterranean Sea, Sir Raman experienced the blues. Armed with a Nicol Prism (to eliminate reflections of the sky) and a small spectroscope (to analyse the colours of the water), he found that the water was intrinsically blue, and not blue from reflecting the sky. Water, much like the atmosphere, also scatters light! He had already written about his discovery before reaching the port of Bombay - a wonderful letter published in Nature, and among my favourite pieces of scientific literature. It was a person spending time in nature, and allowing questions to permeate every corner of his mind. Sir C V Raman had the scientific blues!

"Look at the resplendent colours on the soap bubbles!

Why is the sea blue?

What makes diamond glitter!

Ask the right questions, and nature will open the doors to her secrets"

- Sir C V Raman

As you dive deeper into the water, another property emerges. Water molecules have a vibrational frequency that matches longer wavelengths, such as red light. This results in a compounded effect of light in the reds and yellows of the spectrum being absorbed by the water, and blues being scattered all over the place! What you’re left floating inside of is a spectral world that goes from the lightest blues above to the darkest blues below. In fact, at depths greater than 800m, the ocean is pitch-dark even on a bright sunny day.

"There is a pleasure in the pathless woods, 

There is a rapture on the lonely shore, 

There is society where none intrudes, 

By the deep Sea, and music in its roar: 

I love not Man the less, but Nature more, 

From these our interviews, in which I steal 

From all I may be, or have been before, 

To mingle with the Universe, and feel 

What I can ne’er express, yet cannot all conceal."

- CLXXVIII, Apostrophe to the Ocean by Lord Byron

There are many reasons to be fascinated by the colour blue; my favourite among them is to examine its origin wherever it occurs. This was the story of blue in the sea. Equally fascinating is the story of blue reaching your eyes from the wing of a butterfly!

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About the artist:

Sarah is a silly goose, who waddles through life with a microscope in one hand and a paperback in the other. She loves exploration, science, maps and swears by Lord of the Rings. 🏴‍☠️

About the author

John is a writer, photographer and researcher with a keen interest in spiders. ​ He regularly daydreams of using a calculator, a pencil and a cup of sambar to unravel the secrets of the universe.