Monday 17 September 2012

Science: Why Is The Sky Blue? Tyndall & Rayleigh Effect

Old Man & His Puppy
Image from Locky's English Playground
When you look up and see the blue blue sky, have you ever wondered why it is so blue? Is it because oxygen is blue in colour? Or is it a reflection of the blue ocean? Not at all. It is because of the scattering of light.

Flour suspended in water appears
to be blue because only scattered
light reaches the viewer and blue
light is scattered by the flour
particles more strongly than red.
Image and caption from Wikipedia
What physicist John Tyndall explained in 1859 is that when light passes through a clear fluid with small particles in suspension the blue wavelength of light is scattered much more readily then red.

That means, the image on the left can be seen as blue liquid while red light can pass through the glass and reach the table.

Which goes to explain the fact that since the blue wavelength of the light is shorter (450-495nm) and red wavelength of the light is longer (620-750nm), the shorter the wavelength, the easier it is to be scattered.

Image from NASA
During the time after sunrise and before sunset -- the midday, the sky appears blue because the blue part of the sunlight is scattered by the tiny molecules in our atmosphere, which means the blue light bounces around and thus our eyes interpret the sky as blue in colour.

Tiny molecules in our atmosphere include gases such as nitrogen (78%), and oxygen (21%), argon and water (in the form of vapor, droplets and ice crystals), and many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

Image from NASA
When the sun sets, the sun is lowered to a level that red and yellow lights can now reach directly into our eyes, thus the sun appears as red, orange or yellow to us. (Bear in mind that at the same time, other people from another part of the world might be enjoying their lunch, they are also looking at the same sun, but they will see the sky as blue.)

Typhoon Signal 1 Sunset
Image from Locky's English Playground

Visible Spectrum
Image from Wikipedia

Then again, why doesn't the sky appear as other colours, say violet, because violet (380-450nm) is within the visible spectrum and is even shorter than blue(450-495nm)?

Response curves for the three types of cone in the human eye
Caption and image from University of California, Riverside

Well, it is true that there should be red, orange, yellow, green, blue and violet as well, because our eyes are designed to in a way that we can only see electromagnetic waves typically from wavelength of 390nm (nanometre) to 750nm, which is called the visible spectrum of human. However, the retina cone cells (photoreceptor cells) in our eyes are particularly sensitive to, or peak at, three wavelengths of light, for indigo and violet wavelengths, the cones are stimulated and interpret it as blue sky plus slight red tinge, thus we normally don't see indigo and violet sky in the midday.

Tyndall & Rayleigh Scattering : Glass appear as blue while red-orange-yellow
light passes through
Image from Wikipedia

In 1871, British physicist Lord Rayleigh went on to explain similar scattering of light with the addition of his very own formulas, allowing us to calculate the intensity of the scattered light, and the theory is known as Rayleigh Scattering. If you are interested in the details, do read the links provided.

If not, you may still want to have a good time with your kids trying out the flour suspension experiment above. It can make a little fun family activity.

Next time when you look at the sky, you can appreciate the science in it.

scattering -- (n)   [C usually singular] a small number or amount of things in a particular area 
bear (in mind) -- (vb) [T] to have or continue to have something
electromagnetic wave -- (n) [C] Electromagnetic radiation (EM radiation or EMR) is a form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space. EMR has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation. In vacuum, electromagnetic radiation propagates at a characteristic speed, the speed of light.
visible spectrum -- (n) the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light.
retina -- (n) a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibres of the optic nerve.
cone cell -- (n)[C] photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light.
tinge -- (n) [C] a very slight amount of a colour or of a feeling

Tyndall Effect @ Wikipedia

Why is the sky Blue? @ Department of Mathematics, University of California, Riverside

Blue Sky - Why is the Sky Blue? @ science made simple

Why is the sky blue? @ NASA's The Space Place

"Blue Skies: The Size’s the Limit " @ HowStuffWorks

Visible Spectrum @ Wikipedia

Rayleigh Scattering @ Wikipedia