1

Why ocean appear to be Blue?

I have got two answers

  1. It's surface scatter blue light more
  2. Water absorb red ,yellow, green are absorbed and hence what left is bluish shades.

But ..I got confused, which is the dominant reason?

If I go underwater than "2" one is applied as what I see inside water is only blue color not red that is absorbed.

But I am talking about why I see blue if I am looking 'at' surface of sea? Is that "1" reason??

I am totally confused, please explain it from beginning and in detail (upto high school level)

Qmechanic
  • 201,751
Suresh Chandra Pal
  • 352
  • 2
    One has to remember that water itself isn't colorless but actually is blue. – Triatticus Oct 23 '21 at 14:46
  • But water in glass is not blue – Suresh Chandra Pal Oct 23 '21 at 14:56
  • 2
    Because it's extremely faint, and most often it's other matter in the water that dominates the color on small scales. – Triatticus Oct 23 '21 at 14:59
  • 2
    I think the real question is why is the sky blue? The surface of the sea reflects the sky. – NickD Oct 23 '21 at 14:59
  • I believe Rayleigh Scattering, the reason the sky is blue, might also apply to bodies of water. – electronpusher Oct 23 '21 at 15:32
  • @NickD, Try taking color pictures under water. If you don't use a light, then the deeper you go, the bluer your photos will turn out. If you get down below maybe thirty or forty feet, there won't be any color except blue. That's got nothing to do with light reflecting off the surface. – Solomon Slow Oct 23 '21 at 15:49
  • There is weak absorption of visible light by water: more on the red side and less on the blue side (see https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Visible_region) so sure, if you take color pictures underwater, they tend to be blue. I don't know what the OP meant, but I took it to mean what one sees when looking at the see from above (a boat or a plane), not scuba diving. Sky reflection is the dominant mechanism in that situation. – NickD Oct 23 '21 at 19:39

4 Answers4

6

According to the US National Ocean Service:

The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see. The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water.

gandalf61
  • 52,505
  • But it would be case if I look under water so that when rays enter in water red get absorbed and blue get scattered by drops of water hence appear blue inside , what about when I look at surface !! , does that mean when rays hit surface , red get absorbed and blue reflected??? – Suresh Chandra Pal Oct 23 '21 at 15:58
  • @SureshChandraPal Yes, red light is absorbed, blue light is reflected. – gandalf61 Oct 23 '21 at 16:24
  • Why doesn't that observed in daily life, like water in vessel , swimming pools(happen but...not ocean type dark blue) , buckets , glass etc...it appear transparent , also when we dip any red object it appears red....although red is absorb why water??? I can't get it , explain it In detail in your answer please ....if you can!! With Diagram(rough) of both cases 1. Water in glass 2. In sea – Suresh Chandra Pal Oct 23 '21 at 16:54
  • Also can you answer it....https://physics.stackexchange.com/q/673163/316984 – Suresh Chandra Pal Oct 23 '21 at 16:56
  • And why water is blue underground , due to red get absorb and blue got mostly SCATTERED ( not reflected under water)???? – Suresh Chandra Pal Oct 23 '21 at 16:58
3

What gandalf61 says is true. Another reason is the ocean reflects the color of the sky.

Here is a photo of the ocean from Why the water is blue?

enter image description here

The background is deeper water. It is dark blue, in particular darker than the sky. When you look in the direction of that part of the ocean, the only light you see is reflected from the sky. The surface doesn't reflect much, so you don't get very much blue light. You get no light coming from beneath the surface.

The light part is shallow. Light from the sky reaches the bottom and is reflected back up. So what you see is a combination of a little light reflected from the sky and more light reflected from the bottom.

Here is a photo of glasses of water from a random website (https://www.yourhealth.net.au/wp-content/uploads/2017/06/Glasses-of-water-on-a-wooden-table.jpg)

enter image description here

Like the ocean, a glass of water also changes appearance depending on how much light is reflected or transmitted from below. The surface reflects some light. For the glass on the left, most of the background light comes from the table. For the glass on the right, the angle is just right to reflect a bright light off the surface toward the camera.

Water is very nearly totally transparent to blue light, and fairly transparent to red. Blue light is absorped over hundreds of meters. (Particles in the water can change this.) Red light doesn't get past a few meters. See Why the water is blue?

One difference between the photos is even the shallow part of the ocean is much deeper than a glass. Even a white ocean bottom appears blue because light is absorbed over a long path. For a glass of water, almost all the light from below is transmitted through the glass.

mmesser314
  • 38,487
  • 5
  • 49
  • 129
  • Why doesn't water in glass or vessel appear blue? But colorless – Suresh Chandra Pal Oct 23 '21 at 15:59
  • Glass water can also reflect sky – Suresh Chandra Pal Oct 23 '21 at 16:15
  • Also can you answer it https://physics.stackexchange.com/q/673163/316984 – Suresh Chandra Pal Oct 23 '21 at 16:56
  • There isn't enough water in a glass to absorb much light, so it appears clear. – NickD Oct 23 '21 at 19:48
  • Random photos of glasses of water are very misleading. Not only does the water get color from unknown-colored surroundings, but the glass generally is not colorless either. – Ruslan Oct 23 '21 at 23:16
  • @Ruslan - True, but here getting color from unknown colored surroundings is the point. – mmesser314 Oct 24 '21 at 02:30
  • @mmesser314 , that darker ocean part is only reflecting sky? !! No other source and one more doubt , the rays that are bounced of from surface of water have blue spectrum more ( as red get little bit absorb during bouncing) or does light that bounce is same as white light? ....glass water can also reflect sky but never appear so.....much blue like paint but a little bluish shade??? – Suresh Chandra Pal Oct 25 '21 at 08:43
3

There are multiple effects at play, and how much each contributes depends on the depth of the water, as well as on the direction you're looking at.

If you are looking straight down into the water, then the dominant effects are light absorption and light scattering. If the water is shallow, most of the light reaches the bottom, and is reflected back, so the water appears transparent - unless it's muddy or murky.

If you go further away from the shore, you'll notice that you can still see the bottom, but that its hue is a bit different: it's blue-greenish, and as you go further and further from the shore, it may appear as if you're looking at it through a bluish haze. This is because the light in red-orange-yellow part of the spectrum mostly doesn't reach the bottom - because water molecules absorb it. The blue part of the spectrum, however, partly reaches the bottom, and is partly scattered around (similar to how it's scattered in the atmosphere). The rays that are reflected off of the bottom and manage to reach your eyes is what lets you actually see the bottom. If there's a lot of scattering, the scattered rays come from random directions and thus don't really carry an image - they just produce the blue haze effect.

As you go even further away from the shore, all light is either absorbed or scattered before it reaches the bottom, and all you see is the ocean blue.

Notice also that large waves sometimes have a blue-green tint (especially near the top), even though the ocean from which they arise appears a deeper blue; this is because the thickness of the wave is not enough to eliminate all of the non-blue light.

enter image description here

See also: Ocean Color article from NASA Science website

But now, change your viewing angle - go from looking straight down, towards looking at the horizon. As you change the angle of your gaze, your eyes "catch" the light rays coming from lower and lower angles with respect to the ocean surface. Well, water partially reflects light, and it reflects more light if the angle of the incoming light ray is low (with respect to the surface). E.g. look at this photo - you can see that the water reflects the sky, including the clouds:

enter image description here (free stock photo: source)

In the image below, you can see the lake floor through the water near the bottom of the frame, but as you go towards the other shore you generally see more and more of the reflection, until all you can see are the reflected trees. This is somewhat perturbed by the wavy surface - the waves change the angle of incidence, so in the lower left you also get a discontinuous reflection of the mountains and the sky on the parts of the wavy surface that are angled with respect to the observer.

enter image description here (free stock photo: source)

This is called Fresnel reflection.

See also:
Fresnel Equations
Snell's Window

Filip Milovanović
  • 2,495
0

I think it's simply Rayleigh scattering. Intensity of scattered light is inversely proportional to wavelength to power 4. Violet indigo blue has less wavelength and more scattering power. That is same reason sky is blue .

Sidhi
  • 159
  • I don't know much about scattering .this is just beginner high school explanation. I hope it helps. I will sincerely appreciate suggestions for improvement and a better answer by all senior physicist here. – Sidhi Nov 05 '21 at 16:49