The phenomenon behind sky looks blue, clouds are white and sun appears red at sunrise or sunset

Why Does the Sky Appear Blue: Understanding the Science Behind the Color

Have you wondered why the sky is blue but the clouds are white and why the sun appears red at sunrise or sunset. Why we can see a beam of light filled with dust? Why dangers signal lights are always red in color?

Is it due to reflection of light, refraction of light and dispersion of light?
The answer is none of these. It is due to completely different phenomenon known as scattering of light.

What is scattering of light?

If you switch on torch. Can you see the beam of light?
The answer is no because light is invisible. But when you place your hand or any object then you can see a light because you will see reflection of the light by your hand or object.

Now lets imagine a room is filled with dust and you switch on the torch then you can see a beam of light because the large number of dust particles  scattered the light. So we can see a beam of light.

Similarly we can see a beam of light, when a light can pass through a colloid. But the light cannot be visible when light pass through solution or suspension.

Now do you remember what  do you called this phenomenon?

This phenomenon is known as tyndall effect. This means that tyndall effect is due to scattering of light. Colloids shows tyndall effect but not solution and suspension.
As we discussed that light is scattered by dust but light is scattered by other particles as well such as air molecules and water droplets.

How these particles scattering the light?

If we consider light as a wave then we can say these particles are reflecting the light waves in different directions.
Now if we consider light as a particle made up of photons (these particles are massless) then we can say that these particles are absorbing the photons and re-emitting them in different directions.

Do these particles scatter light in the same way?

The answer is not all particles scatter light in the same way. Before analyzing scattering. First we know about the color of white light. White light is made up of millions of millions of colours. But for simplicity we take as seven rainbow colours or the seven VIBGYOR colours.

How are the different colours scattered light?

It depends on the size of the particles.

How different size particles scattered light?

The atmosphere is a mixture of many different particles such as air molecules, water droplets, and dust molecules and so on. To study the scattering of lights by these different size particles we are going to divide them based on the wavelength of visible light. Wavelength means the length of 1 wave.
As we know the wavelength of visible light is 400nm to 700nm (nm=nanometer)

In the given table particles are divided into two categories.

Smaller than wavelength of visible light	Larger than wavelength of visible light
Air molecule	Water droplet
	Dust particle

 

The air molecule have a very tiny size. There size <0.4 nm or even less because air molecules are made up of nitrogen and oxygen molecules and they are really really tiny. So these particles are smaller than the wavelength of visible light. The water droplets and the dust particles are much higher than the wavelength of visible light. The size of these particles are larger than 1000 nm.

Now when we analyse the scattering of light by larger particles that is the water droplets and dust particles.

In a room of a dust particles the colour of beam of light appears white because dust particles scatter all colours equally, so the beam will appear white.
Now in compare to smaller particles air molecule for example nitrogen and oxygen molecules. These particles are smaller than the wavelength of visible light. Let's say when a white light from the sun falling in air molecules. The air molecules scatters different colours differently. Blue colour scatters more than other colours and red colours scatters least. Because different colours have different wavelengths and shorter wavelength are scattered more than larger wavelength. It means that blue colour has shorter wavelength so it scattered the most, whereas red color has larger wavelength, so it scattered the least.

View this post on Instagram

Infact blue color scattered 10 times more than red color. This is known as preferential scattering. It happens in the particles that are smaller than the wavelength of visible light such as air molecules.

That is the reason sky appears blue.