Monday, December 12, 2005
Why the sky is blue
One of the first questions that a small child will ask their parent is about the colour of the sky. The perpetual beauty of the blue sky wraps the entire Earth, or at least, that half of the planet that is lit by the Sun.
But where does this bottomless blue colour come from?
Myth: Reflection of the blue color of the ocean which covers two-thirds of the planet
Reality: Both the sky and the ocean get their blue color from (white) sunlight being scattered from the tiny molecules that make up our atmosphere and our oceans (Remember your school chemistry 101 -- all particles are made up of molecules and molecules of atoms)
How: Shortly, its due to a phenomenon called 'Raleigh Scattering' propounded by Lord Raleigh. We all know that light gets scattered when it hits any particles and when it hits, is absorbed, and then, is re-emitted. The light might get changed between being absorbed and emitted, changing in (say) brightness, or angle at which it is emitted, or even colour. We have the 'blue' color since the emitted light travels in a right angle when hits any particles and blue color is strong when the light moves right angle
More Explanation:
Now here's the essence of Rayleigh Scattering. First, the re-emitted light does not come out of the molecule in the direction from which it came - instead, it comes out mostly at right angles to its original direction. Second, the "right-angle bending effect" is much stronger for blue light than for red light, so the emitted light is predominately blue. The red light goes mostly straight ahead, and then back out of the atmosphere into deep space - and so you never see it. Third, your eyeball will register seeing a colour only if some light of that colour lands inside your eyeball.
So white light will hit a gas molecule, but bluish light will come out - and at right angles to the original direction. This light will then travel in a straight line until it hits another molecule, and the whole absorb/emit/right angle/blue thing happens again. So the light that came from the sun will hit a gas molecule high in the atmosphere, and then will stutter in a zig-zag fashion down towards the ground. Each time the light hits a molecule, the light comes out mostly at right angles to its previous direction of travel - and each time, it's more blue in colour. Eventually, some light that has been through thousands of these absorb/emit situations with thousands of molecules will land in your eyeball - and then, and only then, do you get the impression of seeing blue.
There you have it - the sky is blue is because of Rayleigh Scattering by the nitrogen and oxygen molecules in the atmosphere. And the ocean is blue because of Rayleigh Scattering by the water molecules that make up the water, as well as reflecting the sky. And that's why, regardless of what Dorothy sings in the Wizard of Oz, blue skies happen in most places, not just over the rainbow.
The Really Deep explanation of why the sky is blue is hard. Albert Einstein wrote, in 1911, that Relativity had to be used to fully understand why the sky is blue.
Source: www.abc.net.au
Reference: Karl Kruszelnicki
But where does this bottomless blue colour come from?
Myth: Reflection of the blue color of the ocean which covers two-thirds of the planet
Reality: Both the sky and the ocean get their blue color from (white) sunlight being scattered from the tiny molecules that make up our atmosphere and our oceans (Remember your school chemistry 101 -- all particles are made up of molecules and molecules of atoms)
How: Shortly, its due to a phenomenon called 'Raleigh Scattering' propounded by Lord Raleigh. We all know that light gets scattered when it hits any particles and when it hits, is absorbed, and then, is re-emitted. The light might get changed between being absorbed and emitted, changing in (say) brightness, or angle at which it is emitted, or even colour. We have the 'blue' color since the emitted light travels in a right angle when hits any particles and blue color is strong when the light moves right angle
More Explanation:
Now here's the essence of Rayleigh Scattering. First, the re-emitted light does not come out of the molecule in the direction from which it came - instead, it comes out mostly at right angles to its original direction. Second, the "right-angle bending effect" is much stronger for blue light than for red light, so the emitted light is predominately blue. The red light goes mostly straight ahead, and then back out of the atmosphere into deep space - and so you never see it. Third, your eyeball will register seeing a colour only if some light of that colour lands inside your eyeball.
So white light will hit a gas molecule, but bluish light will come out - and at right angles to the original direction. This light will then travel in a straight line until it hits another molecule, and the whole absorb/emit/right angle/blue thing happens again. So the light that came from the sun will hit a gas molecule high in the atmosphere, and then will stutter in a zig-zag fashion down towards the ground. Each time the light hits a molecule, the light comes out mostly at right angles to its previous direction of travel - and each time, it's more blue in colour. Eventually, some light that has been through thousands of these absorb/emit situations with thousands of molecules will land in your eyeball - and then, and only then, do you get the impression of seeing blue.
There you have it - the sky is blue is because of Rayleigh Scattering by the nitrogen and oxygen molecules in the atmosphere. And the ocean is blue because of Rayleigh Scattering by the water molecules that make up the water, as well as reflecting the sky. And that's why, regardless of what Dorothy sings in the Wizard of Oz, blue skies happen in most places, not just over the rainbow.
The Really Deep explanation of why the sky is blue is hard. Albert Einstein wrote, in 1911, that Relativity had to be used to fully understand why the sky is blue.
Source: www.abc.net.au
Reference: Karl Kruszelnicki
