Why do colours disappear underwater at depth?

Ten metres of pure water absorbs about 97% of the red light at 650 nanometres. That one number explains most of what divers see. Water soaks up the long, red end of sunlight far faster than the short, blue end, so colours fade in a fixed order as light travels through it. Red goes first, then orange and yellow, while green lasts longer and blue outlasts the lot. The fish and the coral haven't changed colour. The light that would show their colour has been used up on the way down.

Two details tend to catch newer divers out. The fade depends on how far the light has travelled through water, so a subject two metres away looks duller than one at arm's length. Your brain also covers part of the loss while you're down there. That's why the camera usually shows a bluer, flatter reef than the one you remember.

Why does water absorb red light first?

The cause sits inside the water molecule. In a 1993 paper, Dartmouth College chemists Charles Braun and Sergei Smirnov showed that water's colour comes from its atoms vibrating. Red photons carry about the right energy to push the bonds between oxygen and hydrogen into strong vibrations, so the water absorbs them. The main absorption band sits at 760 nanometres, with weaker bands at 660 and 605. Their paper describes this as the only example in nature of colour that comes from molecular vibration rather than from electrons.

The US Geological Survey makes the same point with a swimming pool. Seen from above, the pool looks blue because light bouncing off the bottom crosses enough water to lose its red. The same water in a bucket looks colourless. Colour builds with the length of the path, and a dive puts several metres of water between you and the sun.

At what depth do colours disappear underwater?

Colours don't switch off at a set depth. They fade on a curve, and the curve is steepest for red. The table uses Pope and Fry's 1997 laboratory measurements of pure water to show how much of each colour survives a given distance through it. Seawater also carries plankton, sediment and dissolved material, so the real figures at a dive site tend to be lower.

Colour (wavelength)	Left after 5 m	Left after 10 m	Left after 20 m
Red (650 nm)	18%	3%	under 1%
Orange (600 nm)	33%	11%	1%
Yellow (575 nm)	68%	46%	21%
Green (550 nm)	75%	57%	32%
Blue (475 nm)	95%	89%	80%

Read along the red row and the pattern is plain. Nearly a fifth of 650-nanometre red survives 5 metres, but only about 3% makes it through 10. Orange is down to around 1% at 20 metres, while blue has lost a fifth of its light. PADI trains Open Water Divers to a maximum depth of 18 metres. A newly qualified diver can therefore reach water where natural red and orange are close to gone.

Why does viewing distance fade colour too?

The light reaching your mask has made two trips. It came down from the surface to the subject, then sideways from the subject to you. A buddy's red fins at 10 metres, seen from 5 metres away, are lit by light that has crossed roughly 15 metres of water. Close the gap to half a metre and the second leg almost vanishes. The red comes back with it.

Dive lights and camera strobes face the same maths over a shorter route. A strobe 1 metre from its subject sends light 1 metre out and 1 metre back. Those 2 metres of pure water absorb about half of the red at 650 nanometres. At 2 metres away, the round trip is 4 metres and roughly three-quarters of that red is gone. It works like a gas plan: every metre out gets paid for again on the way back. That's the physics behind the underwater photographer's habit of getting uncomfortably close.

Why does blood look green underwater?

Blood looks red in air because haemoglobin absorbs most other colours and sends red back. Remove the red from the light and the result changes. Data compiled by Scott Prahl at the Oregon Medical Laser Center show how. Oxygenated haemoglobin absorbs about 170 times more light at 450 nanometres, in the blue, than at 650 nanometres, in the red. In the blue-green light left at depth, its weakest absorption falls around 500 to 510 nanometres. The little light that bounces back is blue-green, so a cut tends to leak a dark green or near-black cloud.

NOAA uses the same physics to explain why so many deep-sea animals are red. With no red light to reflect, a red fish looks blackish and becomes hard for predators and prey to see. For a diver, the practical point is simpler. A green cloud from a scrape is still blood, and it deserves the same first aid it would get on the boat.

Does your brain correct for missing colour underwater?

Partly, which is why the photos come as a surprise. Colour constancy is the brain's habit of keeping an object's colour stable when the lighting changes, the reason a white shirt still looks white under a warm bulb. In a 1987 study in Acta Psychologica, Emmerson and Ross asked subjects in green and blue water to report target colours as they'd look in air. Recognition improved when familiar objects gave the brain extra colour cues, and the gain was largest at longer viewing distances. The camera works like an honest debrief. It records the blue-green light that arrives, so an unedited photo from 15 metres tends to look flatter than the dive felt.

That correction is useful for enjoying a reef and unreliable for reading equipment. The brain fills in colours it expects, not colours the light is carrying. If a colour on a gauge, a slate or a cylinder label decides something on the dive, put a torch on it first. Your mask adds its own distortions to size and distance, covered in why everything looks bigger and closer underwater.

Does murky or fresh water change which colours survive?

Yes, because water molecules are only part of the filter. The USGS notes that water rich in phytoplankton usually appears green. Dissolved humus, peat or decaying plant matter can turn it yellow or brown, and suspended soil adds its own tints. All of that absorbs and scatters light on top of what the water itself removes. In green or tea-coloured water, the last colour standing tends to be green or yellowish rather than blue, and the fade happens over a shorter distance. It's one reason a training dive in a highveld dam and a reef dive at Sodwana Bay can look nothing alike at the same depth.

Where do divers see colour loss for themselves?

On a deeper dive, with a colour chart and a torch. PADI's instructor blog suggests colour change at depth as a demonstration for the Deep adventure dive in the Advanced Open Water course. PADI's Adventures in Diving slate includes a colour chart. On the dive, the red and orange blocks tend to read as brown or grey until a torch beam lands on them. The Advanced Open Water Diver course trains divers to a maximum of 30 metres, or to the depth reached on the Deep adventure dive if that was shallower.

Sources

Frequently Asked Questions

Why do underwater photos look blue or green?
A camera records the light that reaches it, and at depth that light has lost most of its red and orange. Your brain adjusts for the missing colour while you're diving, but the sensor doesn't. Shooting closer and adding a strobe or video light shortens the path through water and brings the warm colours back.

Do red filters work on underwater cameras?
A red or magenta filter doesn't add red light. It blocks some of the blue and green, so the red that's left carries more weight in the image. That works in shallow, sunlit water, and it stops helping once there's too little red left to balance.

Why are so many deep-sea animals red?
NOAA notes that red light doesn't penetrate to 100 metres, so a red animal there has nothing to reflect and looks blackish. Red works as camouflage in the deep. Many of those animals also make their own light, which our guide to why the ocean glows at night covers.

Is the ocean blue because it reflects the sky?
Not underwater, where there's no sky in view. The USGS explains that water's blue comes from its molecules absorbing the red end of the spectrum, not from scattering. Surface reflections change how the sea looks from a boat, but a diver at 10 metres is looking through water that has already filtered out the red.

Does salt water remove colour differently from fresh water?
The red absorption comes from the water molecule itself, so it happens in both. The big differences divers notice come from what's suspended or dissolved in the water: plankton, sediment and plant matter.

Why do colours come back on the ascent?
Less water above you means more red reaches the reef and your mask. Pure water lets through more than five times as much 650-nanometre red at 5 metres as at 10 metres. The warmer tones on a safety stop are the same light, with less of it filtered out.

Reviewed September 2026. Devon Ford runs Scuba Evolution, a PADI 5 Star Dive Centre in Glenvista, Johannesburg.