Question: I've been told the water colour is the reflection off the sky but, i've also been told that the sky's colour is the reflection off the sea. Which is correct answer or are they both wrong?

Katie Fala answered on 19 Nov 2019:

That’s a great question! The sky and sea both seem blue, and it is down to the same reason, but they don’t actually depend on each other for their colour. You might have studied a bit about light already and have heard that white light is made up the different colours of the rainbow. Of the colours that the human eye can perceive, red light has the longest wavelength while blue light has the shortest.

Basically as light passes through our atmosphere, the short wavelength of blue light means it gets scattered the most by the nitrogen and oxygen molecules so we perceive the sky to be blue. Similiarly, water molecules in the sea tend to absorb the light of longer wavelengths (reds, oranges, yellows) which means that the majority of light that gets reflected off the water into our eyes is the short wavelength blue light, which is why the oceans generally also seem blue (although the colour can be affected by things like sediments, pollutants, even phytoplankton living in the water!)

Let me know if you have any follow-up questions on this!