The blue colour of the sea is a fascinating natural phenomenon. The sea appears blue due to the way sunlight interacts with the water molecules. In this article, we will explore the science behind the blue colour of the sea and the factors that influence its shade.
How Light Travels Through Water
When sunlight hits the ocean, some of it gets absorbed by the water, while the rest gets scattered. The longer wavelength lights on the red end of the visible light spectrum are absorbed quickly, while the shorter wavelength blue lights are scattered more. This scattering effect makes the colour that is reflected back appear blue.
The water molecules have a weak absorption of red and blue light. But the scattering effect is stronger for blue light compared to red. This causes the blue colour to return back instead of the red, giving a deep blue appearance.
Selective Absorption of Colours
Water molecules absorb light colours selectively based on the wavelength. Red has a wavelength of around 700nm, orange has 600nm, green has 500nm, blue has 450nm and violet 400nm. Out of these, blue and violet colours have the shortest wavelengths.
When sunlight enters water, the red, orange and yellow colours get absorbed rapidly, within 10 metres depth. The blue colour has a longer wavelength so gets absorbed slower. While violet gets absorbed faster than blue. This makes blue colour remain and reflect back more, giving the sea its typical blue appearance.
Scattering of Light
In addition to selective absorption, light scattering also plays a major role. The water molecules scatter short wavelength violet and blue colours more compared to long wavelength red, orange and yellow.
This scattering effect is referred to as Rayleigh scattering. It varies inversely with the fourth power of wavelength. So violet and blue colours scatter almost 10 times more than red. This amplifies the blue colour, causing the strong blue appearance of the sea.
Water Clarity
The clarity and purity of the water body also affects its colour. Clear ocean water appears deeper blue compared to turbid coastal waters. Turbid waters have lots of suspended particles that reflect light rays randomly, reducing the blue colour.
Particles in turbid waters scatter the light more uniformly across the visible spectrum. This creates a muddy brown appearance compared to the strong blue of clearer waters.
Viewing Angle
The sea colour also varies based on the angle it is viewed from. Looking straight down into the sea water, it appears deep blue. But when viewed at an angle, it appears lighter green-blue or turquoise.
At larger viewing angles more light is reflected. The blue colour diminishes and gets blended with greens and turquoise tints. This causes the subtle variations in sea colour along the shores.
Water Depth
Water depth also affects how blue the sea appears. Shallow waters near beaches absorb more red and yellow colours. But insufficient depth prevents absorption of the full spectrum of colours.
So in shallow water more blue light gets reflected, creating a lighter blue-green appearance. As the depth increases, the blue colour dominates more, giving a deep ocean blue appearance.
Time of Day
The sea colour changes subtly based on the position of the sun and intensity of sunlight. During mornings and evenings, the sun is low. Its light has to travel larger distance through the water, allowing more red colours to get absorbed.
This results in deeper hues of blue during the low sunlight hours. In contrast, overhead sunlight around noon reflects back lighter blue or greenish-blue shades.
Water Composition
The mineral content and microbial presence in the water also has small effects on sea colour. Iron-rich waters appear greener, while copper-rich water is bluer. Algae blooms add a greenish tint in coastal regions.
The blue colour mainly arises due to hydroscopic scattering. But the composition slightly modifies the effect by absorbing and reflecting different colours in subtle ways.
Surrounding Environment
The surrounding environment such as seabed and vegetation influence what colours get reflected by the sea. Sand reflects yellow, red and orange hues, while vegetation adds green shades.
Depending on the reflecting colours of the surroundings, the blue sea can take slightly different colour hues in specific coastal regions.
Factors Influencing Blue Colour of the Sea
| Factor | Effect on Sea Colour |
|---|---|
| Water clarity | Clear water appears deeper blue |
| Viewing angle | Straight-on is deep blue, angled is lighter blue-green |
| Water depth | Shallow water is lighter blue-green, deep water is darker blue |
| Time of day | Morning/evening is darker blue, noon is lighter blue |
| Water composition | Minor effects from minerals and microbes |
| Surroundings | Sand, vegetation give subtle colour tints |
Causes of Variations in Sea Colour
Though sunlight scattering gives its characteristic blue colour, the sea does not appear uniformly blue everywhere. There are visible variations in shade from one coastal region to another.
The primary factors influencing these local variations are:
- Water depth – Shallow waters appear lighter blue-green
- Viewing angle – Low-angle makes it greener, while high-angle makes it darker blue
- Suspended particles – More particles make it duller and greener
- Surroundings – Sand, vegetation reflect colours making it subtly different
The purity and salinity of the water also has small effects. So the exact combination of these factors gives the sea a distinctive localised colour.
Blue Colour Significance
The blue colour of the sea has great ecological significance. The specific wavelengths of light penetrating the oceans enable marine life processes.
Plants utilize the blue and green wavelengths for photosynthesis underwater. Blue light reaches deeper compared to other colours enabling coral and vegetation growth.
The blue colour enables camouflage of marine organisms. It helps protect them from predators by blending with the surroundings.
The sea colour even influences heat absorption and impacts climate. Darker blue regions absorb more heat compared to lighter shades.
Unusual Sea Colours
Though blue is the norm, the sea can sometimes take unusual colours such as:
- Milky or turquoise blue – Due to fine glacial sediment particles
- Dark green – Due to algae blooms blocking blue wavelengths
- Brown or yellow – Due to sediments churned up from storms or dredging
- Black – Due to volcanic carbon particles or oil slicks
- Red – Due to algae blooms containing reddish pigments
But these are temporary effects. The blue colour returns when the causes dissipate due to tides and currents.
Protecting the Blue
Human activities are impacting the sea colour through pollution and climate change. Effluent discharge, oil spills and plastic waste make waters appear darker and greener.
Climate change causes algae blooms leading to unusual colourations. Rising temperature and acidity affect the ocean flora and fauna.
Sustainable practices are crucial for minimizing human footprint. It will help maintain the mesmerizing blue colour that has intrigued humankind since ancient times.
Conclusion
The blue colour of the sea arises mainly due to preferential absorption and scattering of sunlight by water molecules. But several factors like depth, viewing angle and water clarity influence the exact shade of blue.
Localized variations in colour have ecological significance and are aesthetically pleasing. Preserving the sea colour requires environmental responsibility to safeguard the oceans.
The blue expanse of the sea and oceans has inspired art, culture and poetry over the ages. Understanding the science behind the colouration further adds to the fascination and respect for marine habitats.