Energy Transmission
in Water — Light
Every diver asks the same question before rolling off the boat: "What's the viz like?" Behind that one word — visibility — sits the whole physics of light in sea water. The ocean is not a window; it is a filter that takes light apart. Sixty percent of the sunlight hitting the surface is gone within one metre. Red is gone by ten. Below a thousand metres there is no light at all, anywhere, ever.
You are diving a reef at Sodwana Bay, 20 metres down, and you scrape your hand on the coral. Blood seeps out — and it is green. Not red. Bright, murky green. Your buddy shines a torch on it and it turns red again instantly. What just happened?
Your blood never changed. The light did. You only ever see an object because light bounces off it into your eye, and the colour you see is the wavelength that object reflects. Blood looks red because it reflects red light. But sea water absorbs light unequally: the long wavelengths — red, orange, yellow — are absorbed first and are completely gone before 10 metres. At 20 m there is simply no red light left in the water for your blood to reflect. So it reflects whatever is down there — blue-green — and that is what your eye receives.
The torch fixes it because a torch is a broad-spectrum artificial light source: it puts the missing red wavelengths back into the water, right there, right then. The blood reflects them, and the colour "returns". Underwater photographers live by this — without a strobe, every photo below 10 m comes back blue.
This topic is the physics behind that moment: what light is, and the five things sea water does to it — penetration, scattering, absorption, attenuation and refraction.
Light is Energy, and the Sea is a Filter
Light is radiated electromagnetic energy from the sun. Sea water transmits only the visible part of that energy — and it takes even that apart, removing it faster with depth and unequally by wavelength.
What light actually is. Light energy is a combination of two changes travelling together: changing electrical impulses moving as waves, and changing magnetic fields forming around those electrical impulses. Each one keeps re-creating the other — the changing electric wave alters the magnetic field, and the changing magnetic field alters the electric impulse. That mutual push-and-pull radiates outwards as electromagnetic energy, which is why sunlight can cross the vacuum of space where there is nothing to travel through. (You met exactly this in Topic 07 as radiation — the one heat-transfer method that needs no medium.)
What reaches the sea. The sun radiates the whole electromagnetic spectrum, from gamma rays to long radio waves. Only a narrow slice of it — visible light, roughly 400–750 nm — is what our eyes detect, and CAPS states plainly that sea water transmits the visible portion of the electromagnetic spectrum. Inside that slice, violet has the shortest wavelength and red the longest.
What the sea does to it. Once light crosses the surface it behaves in five ways: it refracts (bends, because water is denser than air), penetrates (travels down to some depth), is absorbed (converted to heat, longest wavelengths first), is scattered (deflected sideways by suspended particles), and the total of all that loss is attenuation. Understand those five words and you can explain everything from why the open ocean is blue to why a Gqeberha dive after heavy rain is a brown-out.
~ the electromagnetic spectrum — visible light is only that thin bright sliver
Wavelength (λ) = the distance between two successive crests (or troughs) of a wave. Shorter wavelengths than visible light: ultraviolet, x-rays, gamma rays. Longer: infrared, microwaves, TV, FM and AM broadcast bands, long radio waves.
You never see an object — you see the light it reflects
All the light entering your eye has bounced off something in your line of sight. That is light reflection: when light strikes a smooth or shiny surface, it bounces off at the same angle at which it hit. The colour you perceive depends entirely on which wavelength that object reflects back — a yellow nudibranch reflects yellow and absorbs the rest. Take the yellow wavelengths out of the water, and the nudibranch has nothing left to reflect. This single idea is the key to the whole topic.
Five Things the Sea Does to Light
A single sunbeam hits the surface. From that instant its behaviour splits five ways. Tap each numbered node to see what is happening, the definition you must be able to write, and where it shows up in the real ocean.
Take the Dive — Watch the Light Die
This is the topic's central experiment, and you can run it yourself. Drag the slider to descend. Watch two things at once: how much light is left, and which colours survive. Then hit the torch and see the colours come back.
~ drag the slider, or jump straight to a CAPS depth
The five numbers CAPS expects you to know cold
60% of light is absorbed within the first metre. 80% is absorbed in the first 10 metres. On a very clear day in extremely clear water, only 1% of visible light reaches 150 m. No light at all can be seen or recorded below 1 000 m. And the long-wave (red) end of the spectrum is lost within the first 10 m — which is why objects deeper than 10 m appear dark, and red objects appear black.
Where the light goes — it becomes heat
Absorbed light does not vanish; it is converted to thermal energy. That is the definition of light absorption, and it is also the link straight back to Topic 07: the sun's radiation is absorbed by the upper layers of the ocean, warming the surface water and creating the thermocline — a warm surface layer sitting on cold deep water. The reason the sea below 200 m is permanently cold is the same reason it is permanently dark: the energy never gets down there.
Three Words Learners Mix Up
Scattering, absorption and attenuation describe three different things. Exam questions are written specifically to catch you swapping them, so learn them as a set:
| Scattering | Absorption | Attenuation | |
|---|---|---|---|
| What causes it | Suspended particles — silt, sediment, plankton, organic matter | The water itself, acting on particular wavelengths; deeper = more | The cumulative distance light travels through the medium |
| What physically happens | Wave energy is deflected in many directions instead of a straight path | Light energy is converted to thermal energy (heat) | The total intensity of the beam is reduced — absorption plus scattering |
| What the diver sees | Cloudiness, opacity — "bad viz" | Specific colours drain away: reds and yellows first, then everything but blue | Gradual fading to total darkness |
The sunglasses trick — how to remember attenuation
A pair of sunglasses attenuates the sunlight passing through the lenses: less light comes out than went in. Stack a second pair behind them, then a third, then a fourth — each one takes another bite, and eventually zero light gets through. Sea water works exactly like an endless stack of very faint sunglasses. Light attenuation is the loss of light intensity as the distance through a medium increases — and because water never stops attenuating, there is always a depth at which the last photon is used up. In the ocean, that depth is 1 000 m.
How scientists actually measure it
CAPS names two instruments. The Secchi disc — the black-and-white disc you met in Topic 19 and in the Term 2 Paper 1 — is lowered on a marked rope until it disappears from sight; that depth is the Secchi depth, a simple, cheap field measure of attenuation and turbidity. For a proper number, researchers use a light meter (photometer), lowered through the water column to record actual light intensity at each depth. The Secchi disc tells you roughly; the light meter tells you exactly.
Refraction — Why Nothing is Where it Looks
The fifth behaviour happens the instant light crosses the surface, before any of the others get a chance. Light refraction is what happens when light passes from one medium into another of different density.
The result
Light travelling from the pencil to your eye changes speed at the water line. The rays leaving the submerged half arrive at your eye from a different angle than the rays from the dry half, so your brain draws the two halves in different places. The pencil is straight. The light is bent.
The mechanism
Water is denser than air, so light travels more slowly through it. When the ray crosses the boundary it slows down and bends towards the normal (the imaginary line at right angles to the surface). CAPS gives a physical analogy: a cylinder rolling at an angle off hard pavement onto soft sand — the wheel that reaches the sand first slows down while the other keeps going, so the whole thing swings around. Same with a marching band stepping from tar onto mud.
Local connection — how Algoa Bay's gannets beat the physics
Because light bends at the surface, items seen through the surface of water are not positioned where they appear to be. A fish always looks shallower and further away than it really is. Every animal that hunts across the air–water boundary has had to solve this. Bird Island in Algoa Bay hosts the world's largest breeding colony of Cape gannets — tens of thousands of breeding pairs — and they hit the water at around 100 km/h to catch sardines. A plunge-diving gannet aims at the real fish, not the apparent one: it makes its final adjustment underwater, using the momentum of the dive to close the gap. African fish eagles, kingfishers and herons do the same by striking at a steep angle, where the refraction error is smallest, or by getting their eyes as close to the surface as possible before they strike. Human spearfishers on the Wild Coast learn the identical lesson the hard way: aim low.
Why the Sea Changes Colour
Reflection, scattering and absorption together explain the colour of every body of water you will ever look at — and the difference between a 30-metre viz day and a brown-out.
The open ocean
Out in clear deep water there is very little suspended matter, so light penetrates deeply and pure absorption is in charge. The long wavelengths — red, orange, yellow — are absorbed near the surface, leaving only the short blue-green wavelengths to scatter back up to your eye. That is the colour you see.
- Blue light penetrates deeper than any other colour
- The deeper and clearer the water, the bluer it looks
- Sets up Topic 23–24: red seaweeds grow deepest because their pigments use the blue light that gets there
Coastal water
Close to shore the water carries suspended sediment, silt and organic material. Those fine particles reflect and scatter light back out before it can be deeply absorbed — so you see the colour of the suspended matter itself, not of the water. Hence brown after a river flood, green in a plankton bloom, red in a red tide.
- Suspended particles = cloudy, opaque water = poor visibility
- Scattering also cuts the depth light can reach — shallower photic zone
- Straight from Topic 19: turbidity is a sediment story as much as a light story
Warm, clear Agulhas-Current water over coral reef, with very little river sediment and low plankton density.
- Few suspended particles → little scattering
- Light penetrates deep → you can see far
- Still no red below 10 m — bring a torch for photos
Nutrient-rich, plankton-heavy water. False Bay is the warmer, Agulhas-influenced side of the Peninsula — the cold Benguela water is on the Atlantic seaboard — but summer south-easters drive upwelling here too, and the nutrients feed enormous plankton populations. Kelp, swell and fine sediment add more particles to the column.
- Dense suspended matter → heavy scattering
- Light attenuates within a metre or two → "pea soup"
- Same sun, same spectrum — different water
Local connection — reading the viz in Algoa Bay
Divers in Gqeberha can predict the viz before they even reach the water, and they do it with exactly the physics in this topic. After a big south-easter or heavy rain, the Swartkops and Baakens rivers push silt into the bay and swell stirs the sandy bottom back into suspension — scattering spikes and visibility at spots like Riy Banks or the Bell Buoy can collapse to a metre or two. After a few calm days the particles settle out (Topic 19: the finest particles take longest), scattering drops away, and the same sites open up to 8–15 m. The Benguela upwelling on the West Coast keeps the viz permanently low for a different reason: upwelled nutrients drive massive plankton blooms, and every one of those cells is a scattering particle. Nutrient-rich water is productive water — it is just never clear water. That trade-off is why South Africa's best fishing grounds and best diving grounds are on opposite coasts.
In the Lab — the CAPS Investigation
CAPS sets one investigation for this block — "Refraction exploration", with a glass and a pencil — and says light is "more accurately measured by a light meter" than by a Secchi disk. The practical sheet does both, and takes the colour-thief idea onto a dive.
Four investigations · a glass, a phone, a blue screen and a colour card
- 🟢 kitchen Refraction exploration — measure how much water lifts a coin, roll a tin onto a towel to model refraction, and use cooking oil to test whether "density" really is the reason light bends.
- 🟢 kitchen A phone light meter — watch light fade through paper and milky water, then solve the puzzle in the CAPS figures: how can 60 % go in the first metre but only 80 % by ten?
- 🌊 dive The colour thief — make a tomato turn black with a blue screen, and read a diver's colour card from the surface to 30 m. Results supplied.
- 🟢 desk The booklet's questions — the spectrum, Sodwana against Simon's Town, penetration and attenuation, and the view from above the water.
→ Open the Topic 20 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. These eight definitions are examinable and integral to understanding the topic.
Waves of energy formed by simultaneous electrical and magnetic oscillations. It ranges from low-energy radio waves to high-energy gamma rays, and includes the visible light spectrum.
The narrow segment of radiated light waves (400–750 nm) that is visible to the naked eye. Violet has the shortest wavelength, red the longest. Sea water transmits this portion of the spectrum.
In a progressive sound or electromagnetic wave, the horizontal distance between two successive crests or troughs. Short wavelengths carry more energy and penetrate water further.
The measure of how deep light (or any electromagnetic radiation) can penetrate into a material such as a liquid or air. In sea water: 1% at 150 m in ideal conditions; nothing below 1 000 m.
When light strikes the surface of an object it is absorbed and converted to thermal energy. In water, absorption is wavelength-dependent: red, orange and yellow are absorbed within the first 10 m.
As light passes through a medium containing imperfections such as suspended particles (dust or sediment in air and water), the wave energy is deflected in multiple directions instead of a straight path. The amount depends on the particles' size, number and composition.
As light passes through a medium it undergoes absorption and scattering of its energy, resulting in a reduction in its intensity. In short: the loss of light intensity as distance through the medium increases. Measured with a Secchi disc or, accurately, a light meter.
Light passing from one medium to another of different density changes speed. A change from a fast medium to a slower medium bends the ray towards the normal — the imaginary line at right angles to the boundary, not the boundary itself. Air → water bends light; objects are not where they appear.
Also on the examinable list — light reflection
Light reflection: when light strikes a surface that is smooth or shiny, the light will bounce off at the same angle as it hit the surface. Two general-vocabulary terms travel with this topic as well: opaque (a surface that is not transparent or translucent, preventing light from travelling through) and penetrability (the ability to be penetrated). Water thick with suspended sediment becomes opaque; its penetrability drops.
Quick Check
Question 1
Well done!
Next: Important Metabolic Processes — photosynthesis, chemosynthesis and respiration. You already know why it can only happen in the top 150 m.