Electromagnetic spectrum

In this post, we'll talk a bit more about the electromagnetic spectrum. We'll focus on the visible light region as it is easier to illustrate. But the concepts apply to all electromagnetic waves.

Continuous spectrum

We bet you have seen a continuous spectrum already. That's right, the beautiful rainbow.

If you pass the sunshine through a prism, you'll see the rainbow. It is made of multiple colors, including violet, blue, green, yellow, orange, and red. However, you won't see any gap or any separation line between the colors. Colors change to other colors gradually and different colors are blended into each other.

So in chemical language, a continuous spectrum shows lights of all the wavelengths.

Discrete line spectrum

On the other hand, a discrete line spectrum shows lights of certain wavelengths only. There're two different types of discrete line spectrum, and we'll introduce them one by one.

Emission spectrum

This spectrum could be generated by lights shining on a piece of paper, whereas only lights with certain wavelengths exist. The image below shows the emission spectrum generated by hydrogen atoms. On the dark background, only four lines of lights are observable, namely violet, blue, cyan, and red. The area in between the lights is just dark. That shows what we mean by discrete. The lights are separated apart.

Let's use the very simple illustration below to show you how the emission spectrum is generated. We'll first supply some energy to some hydrogen gas. One the hydrogen atoms receive the energy, the electrons from the hydrogen atoms would be excited. They would go from their original position (\( E_1 \)), to a position that's further away from the nucleus (\( E_2 \)). When we stop supplying the energy, the electrons would then go back to \( E_1 \) due to the attraction from the nucleus. Meanwhile, electrons dropping back to their original position would release the extra energy in the form of lights. And that is how we generate the emission spectrum.

There're a few things that we want to explain. First, why would electrons move from \(E_1\) to \(E_2\)? This is because electrons are constantly moving, but restricted in a certain distance from the nucleus, because of the attraction force between the electrons and the nucleus. But once the electrons have more energy, they could go a bit further away, just like those stronger people could go a few steps further in the tug of war.

Another question we'd like to explore is that why would electrons emit lights when they drop back to \(E_1\). Let's think about an apple falling from the tree. It would make some sound, and probably create a dent on the ground. That's when energy is released in the form of sound or is transfered to the ground. The same thing would happen when electrons fall from \(E_2\) to \(E_1\). However, electrons wouldn't make sound or punch a hole in the atom. They would create a photon instead, which carries energy. And that's the light we see.

The third question we have is that why a discrete line spectrum is produced, instead of a continuous spectrum. We'll leave this question to the next post, after we've introduced the other type of discrete line spectrum.

Absorption spectrum

An absorption spectrum looks exactly the opposite of an emission spectrum. What you'll see is that, on a colored background, there're a few dark lines. Absorption spectrum is generated when lights of certain wavelengths are being absorbed.

So how could we "absorb" lights? Let's take a look at the illustration below. We still use hydrogen gas. However, this time we make it very very cold, like freezing. So the electrons are forced to stay in place as if they were frozen. When we turn on the lamp on the left, photons will carry some energy and transfer the energy to the electrons when they pass by the hydrogen atoms. Electrons are now free to move and they would jump over the \(E_2\). After passing through the hydrogen gas, lights are passed through a prism, which separate white light into the rainbow on a piece of paper. However, since some lights with certain wavelengths have been absorbed by the electrons in hydrogen atoms, a few dark lines are seen inside the rainbow, indicating no light at that particular wavelength, or color. So this is how we get an absorption spectrum.

Ground and excited states

We define the original position of the electrons to be the ground state, while excited state referring to the situation of the electrons having more energy than they're in their ground state. So in the above particular examples, \(E_1\) is the ground state as the electrons would stay there when no external energy is supplied. \(E_2\) is then the excited state and electrons could go to the excited state when they have extra energy.

Please take note that the ground state may not necessarily be \(E_1\). It is referring to the state of the electrons when there's no external intervention. However, electrons could be residing in \(E_2\) or even \(E_3\) as their ground states.

Similarly, the excited state could be anything above the ground state. If a lot energy is supplied, electrons can jump from \(E_1\) to \(E_2\) or \(E_3\). In any case, \(E_2\) and \(E_3\) are both regarded as excited states.

Since the position of the electrons is associated with energy of the electrons, we call \(E_1\) and \(E_2\) as energy levels. So we say the electrons are in energy level 1 or energy level 2 etc, instead of saying they're in certain distance from the nucleus.

With all these in mind, we'd like to continue our journey, to answer our previous question: why is a discrete line spectrum produced when energy is supplied to hydrogen gas, but not a continuous spectrum?