Wavelengths Of Light: Deep Sea Travellers

The ocean is a mysterious place, with much of its vast expanse remaining unexplored. One of the key factors limiting our ability to explore the depths is the lack of light. As we descend into the ocean, the light from above is rapidly attenuated, with different colours of light being absorbed at different rates. So, which colours of light travel deepest into the sea, and why?

Water absorbs light differently

In addition to overall attenuation, the oceans absorb different wavelengths of light at different rates. The wavelengths at the extreme ends of the visible spectrum are attenuated faster than those in the middle. Longer wavelengths are absorbed first; red is absorbed in the upper 10 metres, orange by about 40 metres, and yellow disappears before 100 metres. Shorter wavelengths penetrate further, with blue and green light reaching the deepest depths.

This explains why everything appears blue underwater. The colours we perceive depend on the wavelengths of light that are received by our eyes. If an object appears red to us, it is because the object reflects red light but absorbs all other colours. So, the only colour reaching our eyes is red. Underwater, blue is the only colour of light still available at depth, so that is the only colour that can be reflected back to our eyes, and everything has a blue tinge underwater. A red object at depth will not appear red to us because there is no red light available to reflect off the object. Objects in water will only appear in their true colours near the surface, where all wavelengths of light are still available, or if the other wavelengths of light are provided artificially, such as by illuminating the object with a dive light.

Water in the open ocean appears clear and blue because it contains much less particulate matter, such as phytoplankton or other suspended particles, and the clearer the water, the deeper the light penetration. Blue light penetrates deeply and is scattered by the water molecules, while all other colours are absorbed; thus, the water appears blue. On the other hand, coastal water often appears greenish. Coastal water contains much more suspended silt and algae and microscopic organisms than the open ocean. Many of these organisms, such as phytoplankton, absorb light in the blue and red range through their photosynthetic pigments, leaving green as the dominant wavelength of reflected light. Therefore, the higher the phytoplankton concentration in water, the greener it appears. Small silt particles may also absorb blue light, further shifting the colour of water away from blue when there are high concentrations of suspended particles.

The absorption of electromagnetic radiation by water depends on the state of the water. In the gas phase, water has three types of transition that can give rise to the absorption of electromagnetic radiation: rotational transitions, vibrational transitions, and electronic transitions. In liquid water, the rotational transitions are effectively quenched, but absorption bands are affected by hydrogen bonding.

The ocean's zones

The ocean is divided into several zones, based on depth from the surface, light availability, temperature, and topography of the ocean bottom. These zones help in the ease of oceanographic studies. The ocean covers more than 70% of the Earth's surface and contains about 97% of its water.

The first of these zones is the sunlight zone, also known as the epipelagic zone, which extends from the surface to about 200 meters deep. This zone is aptly named as it is well-lit, especially at midday, and is the warmest layer. The abundant natural light generates heat, which penetrates deeper waters due to wind movement. The temperature in this zone can range from 34ºC near the equator to -2ºC near the North Pole. The epipelagic zone is home to photosynthetic organisms such as phytoplankton, which, in turn, support a rich food web.

The second zone is the twilight zone, or the mesopelagic zone, which extends from 200 meters to 1,000 meters deep. This zone receives a small amount of sunlight at midday, but it is generally a dim region. The temperature changes here are the most extreme due to the thermocline, a rapid decrease in temperature, which marks the start of this zone. While there is not enough light for photosynthesis, the twilight zone is home to bioluminescent creatures.

The third zone is the deep ocean, which extends from 1,000 meters to the ocean floor. This zone is further divided into three sub-zones: the midnight zone, the abyss, and the trenches. The midnight zone, or the bathypelagic zone, extends from 1,000 meters to 4,000 meters deep. It is characterized by a consistent temperature of around 39ºF and a complete lack of sunlight. Organisms in this zone, such as certain types of plankton, jellyfish, and squid, are bioluminescent, producing their own light. The abyssopelagic zone, or simply the abyss, extends from 4,000 meters to 6,000 meters deep. It gets its name from the Greek word "abyss," meaning "no bottom," reflecting the ancient belief that the ocean was endless. This zone is extremely cold, with a startling lack of life due to the immense pressure. The deepest part of the ocean is the hadopelagic zone, or the trenches, which extend past 6,000 meters in deep-sea trenches and canyons, such as the Mariana Trench in the Pacific Ocean. Even in these extreme conditions, some life persists, such as the abyssobrotula galatheae, a species of eel discovered in the Puerto Rico Trench.

Light and colour

Light is energy that travels at the universe's fastest speed in the form of light waves. These light waves are electromagnetic energy and, like all electromagnetic energy, they have different wavelengths. The wavelength of light is the distance between two waves. Each colour of visible light has a unique wavelength, and together they make up white light.

The shortest wavelengths are on the violet and ultraviolet end of the spectrum, while the longest wavelengths are at the red and infrared end. In between, the colours of the visible spectrum are ROYGBIV: red, orange, yellow, green, blue, indigo, and violet.

Water is very effective at absorbing incoming light, so the amount of light penetrating the ocean declines rapidly with depth. At 1 metre depth, only 45% of the solar energy that falls on the ocean surface remains. At 10 metres depth, only 16% of the light is still present, and only 1% of the original light is left at 100 metres. No light penetrates beyond 1,000 metres.

The oceans absorb the different wavelengths of light at different rates. The wavelengths at the extreme ends of the visible spectrum are attenuated faster than those in the middle. Longer wavelengths are absorbed first; red is absorbed in the upper 10 metres, orange by about 40 metres, and yellow disappears before 100 metres. Shorter wavelengths penetrate further, with blue and green light reaching the deepest depths.

This explains why everything appears blue underwater. The colours we perceive depend on the wavelengths of light that are received by our eyes. If an object appears red to us, it is because the object reflects red light but absorbs all of the other colours. So the only colour reaching our eyes is red. Underwater, blue is the only colour of light still available at depth, so that is the only colour that can be reflected back to our eyes, and everything takes on a blue tinge. A red object at depth will not appear red to us because there is no red light available to reflect off the object.

Objects in water will only appear in their true colours near the surface where all wavelengths of light are still available, or if the other wavelengths of light are provided artificially, such as by illuminating the object with a dive light.

Water in the open ocean appears clear and blue because it contains much less particulate matter, such as phytoplankton or other suspended particles, and the clearer the water, the deeper the light penetration. Blue light penetrates deeply and is scattered by the water molecules, while all other colours are absorbed; thus, the water appears blue. On the other hand, coastal water often appears greenish. Coastal water contains much more suspended silt and algae and microscopic organisms than the open ocean. Many of these organisms, such as phytoplankton, absorb light in the blue and red range through their photosynthetic pigments, leaving green as the dominant wavelength of reflected light. Therefore, the higher the phytoplankton concentration in water, the greener it appears. Small silt particles may also absorb blue light, further shifting the colour of water away from blue when there are high concentrations of suspended particles.

Light intensity

Within the first 10 metres of water, over 50% of visible light energy is absorbed. Even in clear tropical waters, only about 1% of visible light, mostly in the blue spectrum, penetrates to 100 metres. At 1 metre depth, only 45% of the solar energy that falls on the ocean surface remains. At 10 metres, only 16% of the original light is still present.

The angle at which sunlight strikes the ocean also affects the amount of energy that penetrates the water's surface. Near the equator, the sun's rays strike the ocean almost perpendicular to the surface, allowing more energy to penetrate. In contrast, near the poles, the sun's rays strike the ocean at an angle, resulting in less energy penetration.

Water absorbs light differently depending on its wavelength. Red light, with a longer wavelength, is absorbed more quickly by water compared to blue light, which has a shorter wavelength and higher energy. This is why blue light penetrates deeper into the ocean, giving everything underwater a blue tinge.

The presence of particles in the water, such as phytoplankton, silt, or other suspended matter, also influences light intensity. Coastal waters, with higher concentrations of these particles, appear greener due to the absorption of blue light by phytoplankton and silt. Clearer water, such as that found in the open ocean, allows for deeper light penetration, giving it a clear and blue appearance.

Light and vision

Light is energy that travels at the fastest speed in the universe through light waves. These light waves are electromagnetic energy and, like all electromagnetic energy, they have different wavelengths. The wavelength of a light wave is the distance between two waves. As a light wave's length increases, its energy decreases.

The sun emits electromagnetic radiation, which is represented by the electromagnetic spectrum. Electromagnetic waves vary in their frequency and wavelength. High-frequency waves have very short wavelengths and are very high-energy forms of radiation, such as gamma rays and x-rays. These rays can easily penetrate the bodies of living organisms and can be harmful if absorbed in large doses. At the other end of the spectrum are low-energy, long-wavelength waves such as radio waves, which do not pose a hazard to living organisms.

Most of the solar energy reaching the Earth is in the range of visible light, with wavelengths between about 400-700 nm. Each color of visible light has a unique wavelength, and together they make up white light. The shortest wavelengths are on the violet and ultraviolet end of the spectrum, while the longest wavelengths are at the red and infrared end.

Water is very effective at absorbing incoming light, so the amount of light penetrating the ocean decreases rapidly with depth. At 1 meter depth, only 45% of the solar energy that falls on the ocean surface remains. At 10 meters depth, only 16% of the light is still present, and only 1% of the original light is left at 100 meters. No light penetrates beyond 1000 meters.

The oceans absorb the different wavelengths of light at different rates. The wavelengths at the extreme ends of the visible spectrum are attenuated faster than those wavelengths in the middle. Longer wavelengths are absorbed first; red is absorbed in the upper 10 meters, orange by about 40 meters, and yellow disappears before 100 meters. Shorter wavelengths penetrate further, with blue and green light reaching the deepest depths.

This explains why everything appears blue underwater. The colors we perceive depend on the wavelengths of light that are received by our eyes. If an object appears red to us, it is because the object reflects red light but absorbs all of the other colors. So the only color reaching our eyes is red. Underwater, blue is the only color of light still available at depth, so that is the only color that can be reflected back to our eyes, and everything takes on a blue tinge.

The ocean can be divided into depth layers depending on the amount of light penetration. The upper 200 meters is referred to as the photic or euphotic zone, where enough light can penetrate to support photosynthesis. From 200-1000 meters lies the dysphotic or twilight zone, where there is still some light but not enough for photosynthesis. Below 1000 meters is the aphotic or midnight zone, where no light penetrates.

The wavelength of light that reflects off an object is the color we see. For example, an object that appears red in white light does so because it reflects longer, less energetic red light waves while absorbing the other colors. Red and orange light waves have less energy, so they are absorbed near the ocean surface. Blue light penetrates much farther, so blue objects are more visible in the deep.

Some deep-sea organisms' eyes have evolved to improve their vision in low light. They can be 10 to 100 times more sensitive to light than human eyes, helping them to survive. Meanwhile, some other deep-sea animals have completely lost their ability to see and rely on their other senses.

Frequently asked questions