Every time you see a picture of a swimming pool, they always look so unnaturally blue, as if somebody dyed it or the picture or edited it to look that way.
I was surprised to find that friends and family with pools had that same blue color, except this time I was seeing it in person with my own two eyes. Again, some part of me thought they added some chemical that turned it blue before the guests came over.
Ever since I got a pool of my own and it had this beautiful blue color, and I certainly did NOT dye it, I knew that there was some kind of phenomenon occurring that I didn’t understand.
Think about how water should normally look: clear or colorless. When you run your tap or use the shower, the water that comes out isn’t blue. The water that comes out of your hose isn’t blue either. The water they serve you at restaurants isn’t blue. But the ocean is blue, so clearly there’s some connection. Why is pool water blue?
The water in swimming pools, like the ocean, looks blue because of the way water reacts to the light spectrum. Water absorbs color from light, starting with red first, until eventually leaving only blue light to illuminate the water. The deeper the water is, the darker the blue color is until eventually there is no more light (completely dark).
Not satisfied with that explanation? Keep reading on, and I’ll go over the scientific reason behind why pool water looks blue instead of clear, how you can also make your pool water look blue, and clear up some common misconceptions regarding why pool water is blue.
How does pool water turn blue?
The water that comes out of your hose is clear and colorless, not blue. So why is it that after you fill the pool with clear water, it seems to have turned blue?
A simple explanation is that what you’re seeing is the pool liner’s color behind the water. Most pool owners select blue liners since in their mind that’s the correct color for a pool (have you ever seen someone deliberately pick green liners, or some other unsightly color?).
However, as mentioned above, the real reason your water appears to be blue is due to how water and light interact. It’s the same reason why ocean water appears to be blue as well.
Bodies of water absorb red wavelengths easily, but not blue wavelengths. Remember that light actually appears colorless, but in actuality contains all the colors in the light spectrum. Just imagine all the colors of the rainbow, with red on one side and violet on the other.
When light touches water, red is absorbed the fastest since it’s the first color in the visible light spectrum. Once the red has been absorbed, it will look more blue. The deeper your pool is, the more color gets absorbed, and the darker the blue color will appear. After a certain depth, all of the light will have been absorbed, and it will be pitch black.
Conversely, the shallower a pool is, the more color you can still see underwater. Shallow pools will still appear blue, but less so compared to a deeper pool.
How do I make my pool water blue?
If your pool color is not a beautiful blue, but instead it’s green, reddish pink, or cloudy, then this isn’t a light trick but instead an indication that your pool contains too much algae, dirt, or the pH is just too high.
Test the water
Although common issues with the water quality have identifying symptoms, it’s not always so cut and dry. For example, a green pool doesn’t always mean you have algae growth. Applying the wrong treatment is not only a waste of money, but it could delay you for days, if not weeks that you could have spent enjoying the pool instead.
So the first thing you should do if you suspect something is wrong with the pool water is to use a pool water testing kit to ensure the problem is what you think it is. Water testing kits will allow you to determine the amount of cyanuric acid, calcium hardness, free chlorine, as well as pH and alkalinity levels.
Shock the pool
If the problem is algae, then the solution is to shock the pool. Shocking the pool means to add a large amount of chemicals into the water to rapidly increase the free chlorine levels to kill bacteria, chloramines, and algae.
24 hours after shocking the pool, you must add algaecide to eliminate any remaining traces of algae. There are different types of algae – black, yellow, pink, green – and algaecide will ensure all of them are killed off.
Read the instructions on your algaecide product to know how much to add, as the quantities can vary between products.
Clean and inspect your pool often
Everyday, you should be checking up on your pool and using the skimmer to remove any debris floating along the surface of the pool such as bugs, sticks, leaves, dirt, and so on. If enough debris accumulates on the bottom, use the pool vacuum to clean it up.
You should also make it a habit to test your pool water often. The pH and alkalinity can quickly change due to rain, so you may need to add some baking soda or soda ash to bring the numbers back up. Remember that the pH should be between 7.4-7.6 and the alkalinity between 80-120 ppm.
Inspect the pool pump and ensure it’s working. Backwash the filters and check the pump baskets to ensure they aren’t full. Expect to empty the basket multiple times a week.
Expect to use an algaecide every week. Shock the pool overnight at least once weekly. Add chlorine to the pool every 3-4 days. Test the pool water at least twice a week.
Common misconceptions about blue pool wat
In this section, we go over some common myths that people spread about why their pool water is blue. As mentioned, the real reason is due to how light interacts with water, specifically how water absorbs most of the color, leaving behind blue.
The pools are dyed blue
This just simply isn’t true, especially if you own your own pool and know for a fact that you haven’t added any dye into your water.
The pool walls and floor make the water look blue
This is somewhat true; many pools have blue liners but the floor is often white. If you see the blue walls or blue floor of a pool through the water, it can appear as if the water is blue, but it’s not the water itself that is blue.
Pool finishes are designed such that it enhances the blue coloring of water, which many people now associate with a clean and attractive pool. Understand that this is a psychological trick.
Pool chemicals are responsible for the blue hue
This statement is also indirectly true. If chemicals aren’t added into the water, then bacteria can change the color of the water into green, brown, or even black depending on how dirty it gets.
Chlorine is added into swimming pools as a disinfectant to ensure the water remains clear and clean. By sanitizing the water, chlorine indirectly makes the water look blue since it prevents changes in water color due to bacterial growth that would have made it look a different (unsightly) color.
The sky’s reflection off the water makes it look blue
You can see your reflection off the surface of the water. The sky is blue. So perhaps pool water is blue because it is reflecting the blue sky. However, this doesn’t explain indoor pools, so it’s false.
The bottom line
The reason why your pool water is blue and not clear is due to the way light interacts with a large enough body of water. The deeper the body of water, the more blue it looks and vice versa.
There are many misconceptions as to why water is blue, with people assuming it’s due to dyes, the reflection of the sky along the surface, and the way the pool is finished. There are some truths to them, but it’s not entirely the correct answer.
Whatever the reason, I do have to admit that a blue pool does look very refreshing and cool, and it’s the perfect way to relax on a warm summer day.