Why Is The Ocean Blue

In his travelogue ‘Roughing It’, American writer and humorist Mark Twain described the pristine blue waters of Lake Tahoe, straddling the border between California and Nevada, as the “fairest picture the whole earth affords”. But, it is not only the water of Lake Tahoe that is blue. If you go on an ocean cruise, for most of the time you will be surrounded by iridescent turquoise blue water, although the water may seem to appear green or blue-green at other times. 

However, when you turn on the tap at home and fill a transparent glass with water, it does not look anywhere close to the water of Lake Tahoe or of the ocean. Instead, it is clear as can be. So, why would the ocean water appear blue or green, while a glass of water is colourless?
It is commonly believed that oceans are blue because they reflect the blue sky. This is a misconception. The real reason oceans appear blue does involve reflection, but not because they mirror the sky. The phenomenon of reflecting the sky is prominent only at relatively low angles and when the water is calm and smooth. Others believe that oceans are blue or green because the waters are cleaner and contain fewer pollutants. That can certainly be true and have an impact on the ocean’s colour. 
The main reason oceans appear blue is because of selective absorption and scattering of colours in the sunlight by water molecules. They are good absorber of all colours, except blue, which is mostly scattered or reflected in different directions, including back out of the water towards our eyes. Hence, the ocean appears blue to us.
The depth colours can penetrate the ocean is also responsible for its appearance. Depending on their energy, colours can penetrate to a maximum depth of about 1,000 metres, although only a small amount penetrates beyond 200 metres. The colours that penetrate deeper are the ones with shorter wavelength because they have higher energy. For visible light, the wavelength ranges from 400 (violet) to 700 (red) nanometres. 
The longest wavelength red is least energetic, followed by, in increasing order, orange, yellow, green, blue, indigo and violet. Consequently, the more energetic, unabsorbed, shorter wavelength blue penetrates the ocean deepest, while colours that are absorbed by water molecules, including the high-energy indigo and violet, are quickly filtered out of the water. Thus, the ocean becomes bluer with depth, thereby adding to the blue tint due to reflection.
Oceans do not always look blue, though. Sometimes they look green. Tiny organisms called phytoplankton – the base of several aquatic food webs, are mainly responsible for that. Phytoplankton use chlorophyll, the most important light-absorbing substance in the oceans, to produce carbon by photosynthesis. 
Because of the green pigment of chlorophyll, phytoplankton preferentially absorbs red and blue colours and reflects green. Therefore, the ocean over regions with high concentrations of phytoplankton will appear from blue-green to green, depending on the type and density of the microorganism population there. The more phytoplankton in the water, the greener it is. The less phytoplankton, the bluer it is.
The ocean can be many other colours depending on the amount of sunlight and particles in the water. For example, sand and silt carried into the ocean from rivers, or kicked up from the floor of the ocean by waves and storms, can affect the colours of the coastal waters. This will cause parts of the oceans to look milky brown.
If the water is full of mud, algae or other impurities, the light scattered off these impurities will overwhelm the water’s natural blueness. Moreover, organic detritus such as decayed plant matter and decomposed marine life can complicate the picture, by adding greens, yellows or browns to the hue.
In conclusion, three factors - water molecules, ocean depth and phytoplankton plus intangibles, like pollutants, decayed plant and sea life - determines the colour of the ocean. The same factors apply to other bodies of water, like lakes, ponds and rivers. Lack of depth and absence of phytoplankton will make the water appear colourless, as in a drinking glass.

The writer is a Professor of Physics at Fordham University, New York.

Photos: Writer and Google Image.

Why Is The Ocean Blue

In his travelogue Roughing It, American writer and humorist Mark Twain described the pristine blue waters of Lake Tahoe, straddling the border between California and Nevada, as the fairest picture the whole earth affords. But, it is not only the water of Lake Tahoe that is blue. If you go on an ocean cruise, for most of the time you will be surrounded by iridescent turquoise blue water, although the water may seem to appear green or blue-green at other times. However, when you turn on the tap at home and fill a transparent glass with water, it does not look anywhere close to the water of Lake Tahoe or of the ocean. Instead, it is clear as can be. So, why would the ocean water appear blue or green, while a glass of water is colourless? It is commonly believed that oceans are blue because they reflect the blue sky. This is a misconception. The real reason oceans appear blue does involve reflection, but not because they mirror the sky. The phenomenon of reflecting the sky is prominent only at relatively low angles and when the water is calm and smooth. Others believe that oceans are blue or green because the waters are cleaner and contain fewer pollutants. That can certainly be true and have an impact on the oceans colour. The main reason oceans appear blue is because of selective absorption and scattering of colours in the sunlight by water molecules. They are good absorber of all colours, except blue, which is mostly scattered or reflected in different directions, including back out of the water towards our eyes. Hence, the ocean appears blue to us. The depth colours can penetrate the ocean is also responsible for its appearance. Depending on their energy, colours can penetrate to a maximum depth of about 1,000 metres, although only a small amount penetrates beyond 200 metres. The colours that penetrate deeper are the ones with shorter wavelength because they have higher energy. For visible light, the wavelength ranges from 400 (violet) to 700 (red) nanometres. The longest wavelength red is least energetic, followed by, in increasing order, orange, yellow, green, blue, indigo and violet. Consequently, the more energetic, unabsorbed, shorter wavelength blue penetrates the ocean deepest, while colours that are absorbed by water molecules, including the high-energy indigo and violet, are quickly filtered out of the water. Thus, the ocean becomes bluer with depth, thereby adding to the blue tint due to reflection. Oceans do not always look blue, though. Sometimes they look green. Tiny organisms called phytoplankton the base of several aquatic food webs, are mainly responsible for that. Phytoplankton use chlorophyll, the most important light-absorbing substance in the oceans, to produce carbon by photosynthesis. Because of the green pigment of chlorophyll, phytoplankton preferentially absorbs red and blue colours and reflects green. Therefore, the ocean over regions with high concentrations of phytoplankton will appear from blue-green to green, depending on the type and density of the microorganism population there. The more phytoplankton in the water, the greener it is. The less phytoplankton, the bluer it is. The ocean can be many other colours depending on the amount of sunlight and particles in the water. For example, sand and silt carried into the ocean from rivers, or kicked up from the floor of the ocean by waves and storms, can affect the colours of the coastal waters. This will cause parts of the oceans to look milky brown. If the water is full of mud, algae or other impurities, the light scattered off these impurities will overwhelm the waters natural blueness. Moreover, organic detritus such as decayed plant matter and decomposed marine life can complicate the picture, by adding greens, yellows or browns to the hue. In conclusion, three factors - water molecules, ocean depth and phytoplankton plus intangibles, like pollutants, decayed plant and sea life - determines the colour of the ocean. The same factors apply to other bodies of water, like lakes, ponds and rivers. Lack of depth and absence of phytoplankton will make the water appear colourless, as in a drinking glass. The writer is a Professor of Physics at Fordham University, New York. Photos: Writer and Google Image.