The easiest answer is to say that Earth, when seen from space, looks blue to the human eye because that is the that Earth mainly reflects into the receptive cones of the eye. Students are always shocked to hear that chlorophyll in plants doesn't absorb green light. It reflects green light. That green light is what enters the eye, and that is why you see green. By the same reasoning, Earth looks blue from space because it is reflecting a lot of blue light. Some people say that Earth is blue from space because Earth contains a lot of water. While that does play a part in the answer, it can't be the only reason. If ocean water were critical for a planet to be blue, then Neptune and Uranus would be a different *****. Earth is blue because of what that water does with the light coming into it from the sun. The ocean is made up of water molecules and those moleculesu200a absorb and scatter wavelengths of light. The easiest wavelengths for water to absorb are infrared, ultraviolet, and red light. This is why the sun doesn't provide much warmth to deep waters. As you go deeper the orange and yellow light go away. Blue light carries the most energy, so it has the highest chance of being reflected, scattered, and re-emitted back out into space where it will enter the eye of the human observer, and that person will see blue.
