account 
Oceans and lakes are blue, but if you fill a cup with water, it’s clear! What’s going on?
When we see color, we’re seeing the energy of light that bounces off an object. A basketball is orange because orange light is reflected off it, toward our eyes. The color orange is just a type of light energy, and our brains understand it as color. Other colors of light, like blue, green and purple, are absorbed by the basketball, and we don’t see them. If you shined a blue light at a basketball, it would actually look black because all the blue color is being absorbed, and there is no orange light to reflect back to our eyes.
Water is pretty interesting because it doesn’t really absorb or reflect much color at all! Light passes through it pretty easily.
There aren’t any huge, gaping holes in water that light can just go through, like a car going through a tunnel. Water is jam-packed with tiny microscopic particles. One group of those particles, the electrons, is responsible for causing color (or no color at all!) Electrons are tiny active particles that travel around the atoms that make up water and all substances. The electrons in water aren’t too fond of the energy of visible light. They prefer other types of energy like ultraviolet and infrared, which get absorbed. We can’t see those types of energy with out eyes. Since waters’ electrons aren’t that interested in the light or energy we can see, the light travels by them without much of it being absorbed.
Share-
Further information
Is water blue?Water does absorb a tiny amount of light, but in small amounts, we don’t notice it. Water actually absorbs light that has a reddish color. When there’s a lot of water – like a whole lake full – we notice that red light from the Sun has actually been absorbed. So what color do we see when reddish light is gone?Blue!Blue is the color absorbed the least by water, so it’s the one we see the most. Blue light also scatters more than other light. That’s the reason why the sky is blue. That scattering affects the color of water too.It just takes a big enough sample of water to notice these things. That’s why you won’t see color in the glass you’re drinking. -
Exploration
Blue Water (Through Milk)If you want to make the water in your drinking glass appear blue, you can actually do it by adding something white:Milk!A few drops of milk in a glass of water will make it appear slightly blue. That’s because the white milk is helping to scatter light and the blue light is scattering more than other colors. -
Sources & links
Chaplin, Martin “Water Absorption Spectrum.” 7 June 2009. Water Structure and Science. 28 Jul 2009. <http://www.lsbu.ac.uk/water/vibrat.html> "industrial glass." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 29 Jul. 2009 <http://www.britannica.com/EBchecked/topic/1426115/industrial-glass>.