Gazing at that clear blue sky, have you ever wondered, why the sky is blue, and why not some other color? And in sci-fi movies, they always show, that as soon as the astronauts leave the Earth’s atmosphere, the sky turns all black. Have you ever wondered why is the sky blue and space black?
The question might sound silly but it has perplexed many astronomers. Is it our eye’s delusion or some atmospheric phenomenon or the movie directors are showing it all wrong? Let’s find the answers to all our ‘WHYS’ but first let’s understand how our eyes actually work. You are carrying this convex lens since birth, knowing it a little better won’t hurt.
This small Camera fitted on the upper part of your face is amazing. Behind those beautiful eyes and eyelashes lies a small convex lens, guarded by a gel-like substance called Vitreous body. In simplified terms, your eye works in a similar way to a camera. When you look at an object, light reflected from the object enters the eyes through the cornea. The front of the eye is made up of the cornea, iris, pupil, and lens. The lens focuses the image onto the retina and when light strikes the retina, it’s converted into an electric signal that is relayed to the brain via the optic nerve. The brain then translates the electrical signals into the images a person sees. Now that we know how our eyes work it is easy to understand how we see things and perceive colors.
Secondly, to find the answer to our question, we need to understand the Lightwave behavior. When a light wave encounters an object, the wave is either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object it collided with. These are the different behaviors of Lightwave and this behavior actually decides the color of each and everything we see, be it the sky, the oceans, the clouds, the trees, the mountains, and so on.
Fun fact: The color of the sea is blue, not because the sea reflects the color of the sky but because water absorbs all other colors of sunlight except for blue, and hence the blue color is reflected by the water. So for the Blue color of oceans credit goes to ‘Absorbing behavior of Light.
And the credits for the beautiful blue color of our sky go to the scattering behavior of Light. The Sunlight that falls on earth is called white light, but it is really made up of all the colors of the rainbow and each color has a different wavelength. (And that is why when you pass white light through a prism, a rainbow is formed)
Scattering of Light
When this sunlight reaches our atmosphere, it gets scattered in all directions. The scattering happens when the light wave collides with the particle suspended in the atmosphere. How the scattering will take place depends on the particle size it collides with, but what color would scatter the most depends on the wavelength of color.
Our atmosphere consists of various particles like air molecules, water droplets, dust particles, etc. If sunlight scatters at molecules or particles that are smaller than the white light’s wavelength, then it is called Rayleigh scattering. Such scattering scatters all colors of white light in different proportions. It is responsible for the color of the sky, but why specifically blue color, we will find this later as we move ahead.
Another kind of scattering is Mie Scattering. When the sunlight scatters at particles that are larger than its wavelength, then the light gets scattered more or less uniformly; which means all colors get equally scattered by the water droplet hence we see white color. The white color of the cloud is the result of this phenomenon as clouds are made up of water droplets.
Why the sky is blue?
As Air molecules are small in size, they have a much smaller wavelength than visible light and hence Rayleigh scattering happens when sunlight hits the air molecules. During this scattering, the color with the shortest wavelength would scatter the most and hence will be more visible. And this implies blue color gets scattered the most as clearly Blue color has the highest visibility. But wait this gets more interesting because the color with the shortest wavelength is Violet then why do we see the blue color? The reason the sky doesn’t appear violet is because there isn’t as much violet in sunlight as there is blue and green. There’s definitely a lot of green light in sunlight more than any other color as a matter of fact but it can’t scatter as readily as blue. And that is why the Sky is Blue.
Light wavelength
The blue sky doesn’t look that complicated at all, but surely the reason behind the same is not that simple. Wait till you find out “why the Space is black” I am sure you will be scratching you head in a minute.
Is the sky blue on other planets, too?
It all depends on what’s in the atmosphere! For example, Mars has a very thin atmosphere made mostly of carbon dioxide and filled with fine dust particles. These fine particles scatter light differently than the gases particles in Earth’s atmosphere and hence Mars has a reddish color.
Similarly, If you were on the Moon, which has no atmosphere, the sky would be black both night and day because the moon doesn’t have an atmosphere and hence no particles to scatter the Light. You can see this in photographs taken during the Apollo Moon landings.
Landing on the Moon
Moving on, we now understood why is the day sky blue. And we all know the night sky is dark because of the position of the Sun. When one part of Earth is facing the Sun, the other part appears black because there is no nearby bright source of light. hence, night happens.
Why is the space, Black?
Now the question is if the universe is full of stars, why doesn’t the light from all of them add up to make the whole sky bright all the time? This is a paradox because of the apparent contradiction between our expectation that the night sky should be bright and our experience, that it is black and is called Olbers’ Paradox :
The question sounds quite simple but the answer to this simple question is not that simple there are only theories behind this mystery and nothing is proven yet.
The universe is vast and the distance between the stars is extensive. This distance impacts the intensity of the light emanating from stars and makes them appear dimmer as a result. To make matter worse, light originating from the universe’s furthest galaxies struggles to reach Earth because of a phenomenon called redshift. The light from the most distant galaxies attempts to reach us, but some of it eventually arrives in the form of infrared light, which the human eye can’t really see.
What is the redshift?
There is a theory that suggests that the universe is expanding and galaxies are moving far from each other at great speed. When the star is moving away from the earth, its light is shifted to lower frequencies on the color spectrum towards the orange/red/infrared /microwave/radio end of the spectrum. A lower frequency shift is called a “redshift“
When stars move away, their light wavelength gets longer, and the longer the wavelength, the lower the frequency, and hence the less visible light, until there is no light that we can see as it gets converted into infrared light due to longer wavelengths. The same thing happens with sound waves. In the case of sound waves, this effect is called the Dopplers Effect. Have you ever observed when an ambulance passes you by, there is high pitched siren sound when it is coming toward you and the sound pitch gets lower as it gets farther away from you? The same thing happens during redshift in the case of Lightwaves. As the light movies farther its wavelength gets longer and turns into infrared ray which is not visible to humans’ eyes.
Another reason which adds up to making space less bright is that the universe is not infinitely old. The universe is between 10 to 15 billion years old. And that means that the maximum distance from which we can receive light is between 10 and 15 billion light-years away. So even if there are more distant galaxies, their light will not yet have had time to reach us. The light from stars farther away didn’t reach us yet and hence couldn’t contribute to making the sky bright. Even the astronauts on the moon experience a 1.5-second time delay in their communications as the radio signals (which are a form of light) take 1.5 seconds to travel round-trip between Earth and the moon.
Fun Fact: Whenever you look up in the sky, you are looking at the past.
Because, as some of the stars are millions of light-years away from us, the picture you see today is actually a million years old. Even sunlight takes 8 minutes to reach earth. And, If aliens are watching our planet through some powerful telescope, from Stephan’s quintet (cluster of 5 galaxies) some 250 million light-years away from us, what they are seeing on Earth at this very moment, here and now, is the early ancestors of Dinosaurs.
So, the simple question ” Why is the sky blue and space black turned out to be not so simple. But next time when you will be stargazing and wondering about this amazing but mysterious creation, you will have one less thing to wonder about as you already have an answer now.
Stay Curious!
