Where does light and colours come from?
Take a look at an apple. In the day light this apple will look red. But why? The answer starts with the fact that sunlight is made up of all colors of light with different wavelengths and power densities and each wavelength is a different color. You can see the different colors in white light when is passes through a prism and the light refracts (bends) and separates into different colors. Typically light with shorter wavelength such as the color blue will have higher power density and light with longer wavelength such as the color red will have significantly less power density. The apple looks red because molecule in it skin interact with the various wavelengths of sunlight. Most of these wavelengths are absorbed by the apple except red which is reflected. Our eyes detect the reflected red light and our brain register that we are seeing a red apple.
The ocean appears blue because of the light from the sun.
I’ve conducted an experiment where I dive down into the ocean with a red apple in my hand. I compared the color of the apple at different depth of the ocean. In one meter, the apple still look red. However when I descend deeper into the ocean at around 35 meters deep, the apple appear to be black. Why is that? The apple did not really change in color. At the surface the apple look red because it reflect red wavelength but at 35 meter it does not appear to be any red wavelength for the apple to reflect and it absorb all the other wavelengths so its look black.
The ocean appear to be blue because red, orange and yellow are absorbed more strongly by the water compared to blue. Therefore, when light travels into the ocean, it is mostly the blue that gets reflected into our eyes.