We are still focused on our quest of understanding Why Is The Sky Blue? We understand white light is the sum of all colors of light, and the sunlight hits the atmosphere and scatters blue energy, but we don't know why. So let's examine the Earth's atmosphere.
What we know of as the "atmosphere" is simply trillions of molecules, mostly Nitrogen (78% N2) and Oxygen (21% O2). The molecules rise in the air because they are warmed by the sun and move randomly (this means they possess kinetic energy), and don't travel infinitely far into space because they are bound to the Earth via gravity.
Let's take a look at one of these molecules, let's say Oxygen (O2):
Figure 1. An Oxygen Molecule (O2).
The oxygen molecule is two oxygen atoms joined together by molecular forces. The center of the atoms are the positively charged nucleus, represented by the blue circles in Figure 1. Surrounding the nucleus is a "cloud" of electrons, which are free to move as they please. The electrons repel each other so they keep their distance, but are held to the positively charged nucleus (like charges repel, opposites attract).
The Electric Field
Now, the light waves we have been discussing contain a travelling Electric Field (see the link for a thorough tutorial of the Electric Field). The Electric Field is essentially the force that causes positive charges to attract to negative charges. This force is oscillating at the frequency of light. The Electric Field (which carries energy of the lightwave) is perpendicular to the direction of travel of light. This is illustrated for blue and red light as shown in Figure 2:
Figure 2. If Light Travels to The Right, the Electric Field will be Vertical.
Note that the color is determined by how quickly the Electric Field oscillates. The red light is oscillating at a slower rate than the blue light. This is all that physically distinguishes red and blue light. Recall that they both travel at the exact same speed, the Electric Fields just change at different rates.
Here is a summary of what we need to know about the Electric Field:
The Interaction of the Electric Field and Molecules
Let's now look at what happens when sunlight hits an Oxygen molecule:
Figure 3. Light and the Associated Electric Field Acting On an Oxygen Molecule.
Figure 3(a) shows an undisturbed Oxygen molecule. Now imagine a light wave is travelling to the right as in Figure 3(b). The Electric Field happens to be pointed upwards at this moment. Electric Fields push on the charges (electrons and protons) shown in the oxygen molecule. Now, the protons are part of the massive nucleus, so they won't be moved by the electric field of the lightwave. However, the electrons are much more free to move. Hence, the electric field disturbs the balance of the charges, and shifts the electrons as shown in Figure 3(b).
Remember that the light wave is oscillating (changing with frequency) in time as it travels. This means a fraction of a second later, the electric field will be pointing the other way, as shown in Figure 3(c). This will disturb the electrons of the molecule as shown.
Essentially then, when a lightwave hits a molecule in the atmosphere, the electrons start moving around. And the flow of electrons is known as electric current. What we have just created is a very small antenna. Each molecule disturbed by the lightwave will act as a small antenna, and this produces re-radiation, or the scattering of the light's energy.
On this page we learned that the Earth's atmosphere is made up mostly of Oxygen and Nitrogen molecules. We learned that a lightwave has an associated electric field, and this electric field disturbs the electrons of the molecules, causing them to oscillate just as the electric field oscillates (remember, how fast the lightwave oscillates is the frequency of the light - and this tells us the color of the light).
Each molecule then becomes a tiny antenna, which re-radiates and scatters the light from the sun. Next we will want to understand why this re-radiated energy is more blue than anything else. For us to understand that, we will need to learn some things about antennas. Go to the Antennas page when you are ready.
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1. Why Is The Sky Blue?
2. White Light