The sky is blue. Why is that? This is a fact that is so natural that most of the time we do not question. Why is the ocean blue as well? Why is the waterfall white? Why are leaves and grasses green? Why is space black? To answer these questions, we need to understand what it means to be able to see physically and biologically.
We see things when light enters our eyes. Light is projected onto the retina, a layer that has cells sensitive to light, and the cells transmit the stimulus on the retina into signals. These signals are passed through the optic nerve to the brain. Our brains then judge this as ‘seeing’ things.
When we see an object, the light we are receiving into our eyes is reflected by the objects we are seeing. If you are seeing a red rose, the rose petals are receiving various spectrums of light but reflecting only red lights. The lights except for red are absorbed by the petals, so they are not coming toward our eyes. This is how we see “red” rose petals.
In the atmosphere there are a lot of air particles, and they reflect only blue light. That is why only blue light is reflected across the sky to our eyes, and it is the reason why the sky appears blue. On the contrary, if you go to the moon, the sky is black, and you can see stars even during the daytime because there is no air on the moon.
This is the mechanism for seeing things, and for us to see things, it takes a fraction of a second for light, hitting an object and reflecting off it until it reaches our eyes. Interestingly, What this means is, everything we see is “The past”.