Why is the sky and sea blue?
1. Outer space contains virtually no gas/dust to scatter
Outer space is black.
2. The sky is blue because that's the colour of air (mostly
nitrogen), which is not totally colourless. You can see this effect
looking at distant (10 km-plus) mountains. They have a blue tinge
(caused by the intervening air between you and the mountains) known
as atmospheric perspective.
3. If the sky's colour was due to reflection from the oceans,
then it would be white above Antarctica, which is snowy white.
4. If the ocean was blue because it reflects the sky, why does
it still look blue-ish when under a totally clouded-over sky?
5. Why blue? Light scattering by the atmosphere (Rayleigh
scattering) has most effect on the most energetic light. The most
energetic light is that with the highest frequency/shortest
6. So, while all light is scattered, blue light is scattered the
most, so the atmosphere "captures" more of the blue light, letting
more of the red light through, and the atmosphere is coloured blue
by that "captured" light.
7. If there were *no* selective scattering, and all wavelengths
were scattered *equally*, the sky would appear white.