Light's River: Understanding Light's Unique Travel Behavior

Light travels in a way that shares similarities with the flow of a river. When light passes through different mediums, it can attenuate, disperse, bend, spread, or continue flowing. For example, when light travels from air into water, it slows down and changes direction, bending more towards what is known as the normal line. This phenomenon is called refraction. In a similar way, rivers bend and change direction as they flow across the earth, carving out new paths and directions. The movement of light through certain mediums can also be observed as a branching flow, where the path of the wave splits into smaller channels, akin to the way a river delta breaks up into smaller rivulets and tributaries.

Light travels slower in water than in air

The speed of light in a vacuum is a constant c, or 299,792,458 metres per second. Nothing can travel faster than this speed. However, when light travels through a medium other than a vacuum, such as water or air, it will be slower.

The speed of light in water is considerably less than the speed of light in a vacuum. Water has a refractive index of about 1.3. The phase velocity of light in a medium with refractive index n is vlight = c/n.

The passage of light through water is an example of a phenomenon called "branching flow". Branching flow occurs when the path of a wave splits, breaking up into smaller channels like the branches of a tree. This can happen when the structure of the medium is random, and the spatial variations in the structure are larger than the wavelength of the flow.

The behaviour of light in water has important implications for the world's ocean systems. A certain amount of incoming light is reflected away when it reaches the ocean surface, depending on the state of the water. If the water is turbulent, with many waves, more light will be reflected. Once light penetrates the surface, it is refracted due to the fact that light travels faster in air than in water. Most of the visible light spectrum is absorbed within 10 metres (33 feet) of the water's surface, and almost none penetrates below 150 metres (490 feet) of water depth, even when the water is very clear.

Light can be detected up to 1,000 meters in the ocean

Light is crucial for ocean systems for several reasons. It provides the energy for ocean currents, wind-driven waves, and the thin layer of warm water near the ocean's surface that supports most marine life. The transmission of light in seawater is essential to the productivity of the oceans. Marine plants with chlorophyll capture the visible wavelengths of light and make their own food through photosynthesis. The organic molecules produced during this process are an important energy source for small organisms at the base of the marine food chain.

The uppermost, sunlit layer of the ocean where 70% of the world's photosynthesis takes place is called the euphotic zone. This zone typically extends to a depth of 100 meters. Below this, from 100 to 1,000 meters deep, is the disphotic or "twilight" zone, where there is a dim light. Some animals can survive here, but no plants, as there is not enough light for photosynthesis.

The ocean's depth and the scattering and absorption of light by water molecules and particles limit how far light can travel below the ocean's surface. Greater abundances of solid particles in the water will decrease light penetration. Therefore, water near the seashore that is more turbid due to particles will show a decrease in light transmission, even in shallow water.

Light behaviour in water is called refraction

The refraction of light in water is an important phenomenon that influences the transmission of light in the ocean. The ocean is divided into zones based on depth and light level, with the euphotic zone being the uppermost sunlit layer where the majority of photosynthesis takes place. Below this is the disphotic zone, where some animals can survive but no plants as there is not enough light for photosynthesis.

The bending of light by refraction is also responsible for the scattering and absorption of light in water. Different wavelengths of visible light are scattered and absorbed to varying degrees, with shorter wavelengths like blue and green penetrating deeper into the ocean, while longer wavelengths like red and orange are absorbed within the first 50 meters.

The behaviour of light in water, including refraction, has been studied by scientists using various methods and instruments. One simple method involves using a Secchi disk, a white plate attached to a rope that is lowered into the water until it disappears from view, providing an estimate of light penetration depth. More advanced instruments like the nephelometer and transmissometer measure light scattering and attenuation, respectively.

A recent study by physicists observed light behaving like a river for the first time. By using a laser and a soap bubble, they were able to observe the branching flow of light as it split into smaller channels, similar to the way a river delta divides into tributaries. This research has implications for optofluidics and could be used to explore various physical phenomena, including aspects of general relativity.

Light is essential to the productivity of the oceans

The transmission of light in seawater is also crucial for photosynthesis, a process by which chlorophyll-bearing marine plants, known as phytoplankton, make their own food. These phytoplankton are photoautotrophs, harvesting light to convert inorganic to organic carbon. They then supply this organic carbon to heterotrophs, which are organisms that obtain their energy solely by consuming other organisms or their by-products. Heterotrophs include bacteria, zooplankton (floating animals), nekton (swimming organisms, including fish and marine mammals), and benthos (organisms that live on the seafloor).

The uppermost, sunlit layer of the ocean, where 70% of the world's photosynthesis takes place, is called the euphotic zone. This zone generally extends to a depth of 100 meters (330 feet). Below this is the disphotic zone, where some animals can survive, but no plants as there is not enough light for photosynthesis. The aphotic zone is the layer of the ocean where no light penetrates at all, and it makes up over 90% of the entire ocean area on Earth, with depths of more than 1,000 meters (3,300 feet).

The availability of light in the ocean is affected by several factors. Firstly, the state of the water surface influences light reflection, with calmer waters reflecting less light and turbulent waters with many waves reflecting more. Additionally, light is scattered or absorbed by solid particles in the water, with greater abundances of particles decreasing the depth of light penetration. This is often seen in coastal areas, where water is more turbid due to particles brought in by river systems and biological production by microorganisms. The colour of light that penetrates the ocean also varies, with longer wavelengths of red, yellow, and orange light reaching shallower depths, and shorter wavelengths of violet, blue, and green light penetrating further. This is why deep, clear ocean water often appears blue.

In conclusion, light plays a critical role in the productivity of the oceans, driving ocean currents, supporting marine life, and enabling photosynthesis in the euphotic zone. The availability and penetration of light in the ocean are influenced by factors such as water conditions and particle concentration, ultimately shaping the distribution of marine life.

Light transmission in water can be measured using a Secchi disk

The Secchi disk is a useful tool for studying aquatic habitats and is particularly valuable for volunteer lake monitoring groups and citizen science initiatives. It is easy to use and requires little equipment or expertise. The disk is mounted on a pole or line and slowly lowered into the water. The depth at which the disk disappears is recorded, and this is the Secchi depth. This depth is reached when the reflectance of light equals the intensity of light backscattered from the water.

There are some considerations when using the Secchi disk. Firstly, it is important to standardise the method as much as possible, as there can be variations between different users. Measurements should be taken at a similar time of day, between 9 am and 3 pm, with the best results between 10 am and 2 pm. The same observer should take measurements consistently, and it is recommended to take measurements in calm waters, away from direct sunlight, to minimise reflection. Additionally, it is important to consider external factors such as sunlight intensity and waves, as these can impact the depth at which the disk disappears.

While the Secchi disk is a useful tool, it does have some limitations. It does not provide an exact measure of transparency, and there can be errors due to the sun's glare on the water. Additionally, different people may have varying levels of visual acuity, which can affect the depth at which they can see the disk. Other methods, such as turbidimeters and transmissometers, can provide more scientifically accurate measurements of water transparency. Nevertheless, the Secchi disk remains a valuable tool for measuring light transmission in water due to its simplicity, accessibility, and applicability in a wide range of environments.

Frequently asked questions