In this explainer, we will learn how to determine the composition of a material from the features that appear in the spectrum of light coming from it.
Light is a spectrum. For visible light, its color depends on its wavelength, , as shown in the diagram below.
This spectrum can occur when pure white light, made up of all visible wavelengths, is shone through a prism. Sunlight is pretty close to being pure white light and will create a spectrum like this. Such an unbroken spectrum is called a continuous spectrum. An unbroken spectrum shone through a prism is shown in the figure below.
Not all light sources emit pure white light. For example, halogen light bulbs emit much more red and yellow light than light of other wavelengths and so appear to give off warmer-colored glows than sunlight. The wavelengths of light these light bulbs emit is a continuous spectrum, even though it is mostly red and orange light. This spectrum is shown in the graph below.
The -axis of this graph is the amount of light being emitted, the intensity, and the -axis is the wavelength of light, corresponding to color.
Halogen light bulbs emit light due to blackbody radiation. Blackbody radiation occurs when an object has a temperature, causing it to emit light. Blackbody radiation is a continuous spectrum, meaning it may vary in intensity in certain regions, but it still emits at all wavelengths.
Pure hot gases can also give off light, like those found in neon tubes. Unlike blackbody radiation, the light in these tubes is not a continuous spectrum. These gases only emit very specific wavelengths of light, as shown in the diagram below.
This hot gas is releasing photons, no outside light source is present. The reason it emits photons of specific wavelengths is because the photons can only have specific energy. Recall the equation to express the energy of a photon, , where is Planck’s constant, is the speed of light, and is the wavelength of light.
This energy is related to the difference in electron energy levels. When an electron transitions downward, it emits a photon with an energy nearly equal to the energy difference of the electron energy levels. An electron transitioning from the 2nd energy level to the ground state, and its subsequent emitted photon, is shown in the figure below.
If the energy of the ground state is and the 2nd energy level is , then we can relate the difference in electron energy levels with the energy of the emitted photon with the following equations:
Hot gas is better at creating emission spectra. A higher temperature means a higher general level of energy, meaning less electrons will be in a ground state. Hotter gases will more readily emit photons.
Since every element has its own unique electron energy levels, it means that every element has its own unique emission spectra, as seen in the diagrams below.
Some lines between elements can be very close to one another, like in the violet light region in hydrogen and helium. This is why it is important to look at the entire spectrum in order to verify their uniqueness. Comparing emission spectra of known elements can be used to identify unknown gases.
Let’s look at an example.
Example 1: Identifying an Unknown Gas Using Emission Spectra
A scientist has a sample of an unknown gas. In order to identify the gas, he looks at the spectrum of visible light emitted from it when it is heated. This is shown in the figure. Also shown in the figure are the emission spectra of five pure, gaseous elements. Which of the five elements is the unknown gas?
Let’s start by looking where there are definitely no emission lines in the other gases. This unknown gas has no emission lines at around 440 nm or below, so it cannot be helium, oxygen, or argon.
Xenon appears to have some lines that match up, but comparing the lines to neon shows that they match up completely. The unknown gas is thus neon, which is choice E.
Emission spectra are not the only kind of spectra that are useful in identifying gases. There are also absorption spectra, which are the opposite of emission spectra. Instead of showing the photons being emitted by the electrons of a gas, it shows where they are being absorbed.
The black lines on the absorption spectra indicate regions where there is no light of that specific wavelength. Absorption spectra are produced when light passes through a gas on its way through a prism, like in the figure below.
The reason some very specific portions are missing is because the gas is actually absorbing the light of those wavelengths, preventing it from showing on the spectrum. The gas does not absorb the other wavelengths of light that are let through.
This gas is absorbing only certain wavelengths of light, meaning photons with specific energies. Recall that the equation to express the energy of a photon is
This photon energy is related to the difference in electron energy levels. In order for an electron to transition upward, it must absorb a photon with a nearly equal energy to the difference in the energy levels. An electron transitioning from the ground state to the 3rd energy level, and the photon it absorbed to do so, can be seen in the figure below.
If the energy of the ground state is , and the energy of the 3rd energy level is , then we can relate the difference in electron energy levels with the energy of the emitted photon as follows:
Just as electrons emit photons of specific energies to transition downward, electron energy level transitions also require photons of specific energies to transition upward.
Cooled gas is better at creating absorption spectra. A lower temperature and thus general energy means more electrons are at a ground state. This means the gas has more chances to absorb the light than if some electrons were already at a higher energy level.
Each element has its own unique absorption spectra, just like it has a unique emission spectra. This means they can be used to identify unknown gases. Some example spectra of pure gases are shown in the diagrams below.
Let’s look at an example.
Example 2: Identifying an Unknown Gas Using Absorption Spectra
A scientist has a sample of an unknown gas. In order to identify it, she shines a continuous spectrum of white light through the gas and observes which wavelengths of light are absorbed by it. This is shown in the figure, as well as the absorption spectra of five pure, gaseous elements. Which of the five elements is the unknown gas?
We are looking at several absorption spectra here, and we wish to find the one that most closely matches the unknown gas.
Helium cannot be it, since the unknown gas has no absorption lines in the region of violet light ( nm). For the same reason, neither oxygen nor argon can be it either, since they have absorption lines there.
Our choices are down to neon and xenon. Neon appears to match some lines at first, especially in the regions of blue and red light, but not all of them. Xenon on the other hand is a perfect match; its lines are the same.
The answer is thus choice C, xenon.
When an electron absorbs the energy of a photon, and so becomes excited, it is not able to stay in that higher energy level for long. Excited electrons eventually transition to lower energy levels, releasing the energy difference between the energy levels as a photon.
Some absorption lines are wider than others. The longer an electron is held at a higher energy level, the narrower the absorption line that is produced.
This means wider lines indicate a region where the electron transitions take a short amount of time to occur. Conversely, thinner lines indicate regions where electron transitions take more time to occur.
Let’s look at an example.
Example 3: Finding the Absorption Line Width for Xenon
The figure shows the absorption spectrum for xenon between 400 nm and 420 nm. Which of the absorption lines marked on the diagram has the greatest width?
- Line A
- Line B
- Line C
- Line D
- Line E
- Line F
The various line lengths here indicate how long an electron stays in a higher energy level before transitioning downward, with thinner lines meaning they take a longer time. Looking at these lines, it is apparent which one is the widest without having to measure; it is line E.
The associated electrons of line E that are absorbing photons thus spend the shortest time in higher energy levels.
The correct answer is choice E, which is line E.
The use of emission and absorption spectra is helpful for identifying unknown gases, but unknown gases are rarely made up of one element. When looking at a mixture of gases, the spectrum lines for all the gas constituents show up, as seen in the diagram below.
The diagram above shows the individual emission spectra of oxygen and helium, and their combination in the center. The mixture contains all the emission lines of both oxygen and helium.
Similarly, an absorption spectrum’s absorption lines will all be present in a gas mixture, as shown in the diagram below.
Let’s look at some examples.
Example 4: Determining Gas Components Using Absorption Spectra
An astronomer looks at the spectrum of light from a distant star. Between Earth and the star is a large cloud of dust and gas. The star emits continuous-spectrum white light, but some of the light is absorbed by the cloud. The figure shows the spectrum of light that the astronomer observes as well as the absorption spectra of several pure elements. Which of the five elements shown does the interstellar cloud contain?
- Hydrogen and helium
- Hydrogen and oxygen
- Oxygen and nitrogen
- Oxygen and carbon
- Hydrogen, helium, and nitrogen
The observed spectrum is made up of several different gases. To find out which ones, we need to look at the absorption lines and match them up as closely as possible to the available gases.
We should start by looking at what general regions the interstellar cloud’s spectrum does not contain any absorption lines at all. It has a large gap between 520 to 580 nm with no bands, so any gases that have bands there are not in the cloud. This eliminates carbon, nitrogen, and oxygen as possible constituents of the gas.
This leaves hydrogen and helium. The absorption lines match up exactly with those in hydrogen and helium. In particular, the interstellar cloud contains the single absorption line at around 585 nm just like helium does, which is only shared by nitrogen (which we already eliminated).
The answer is choice A, hydrogen and helium.
Let’s look at an example of this using emission spectra now.
Example 5: Determining Gas Components Using Emission Spectra
A scientist has a gas canister that contains a mixture of unknown gases. In order to identify which gases are in the mixture, she looks at the spectrum of visible light emitted from it when it is heated. This is shown in the figure. Also shown in the figure are the emission spectra of several pure, gaseous elements. Which of the five elements does the mixture contain?
- Hydrogen, helium, and nitrogen
- Hydrogen and argon
- Helium, hydrogen, oxygen, nitrogen, and argon
- Oxygen, helium, and hydrogen
- Helium, oxygen, nitrogen, and argon
This gas mixture has a lot of emission lines, but we just need to find one difference to rule out a particular gas as being part of it. We cannot start by observing where the unknown gas mixture has no emission lines, as it pretty much covers the entire spectrum. So let’s compare each gas one by one.
Hydrogen’s lines in the region of purple light seem to match up, but the lines around 485 nm and 655 nm just barely do not. This gas likely does not contain hydrogen.
For helium, we see that some of the green lines around 500 nm match up exactly and are not present in any other gas, so it definitely contains helium.
All of oxygen’s lines appear to be present within the gas mixture, so it contains oxygen.
The gas mixture’s thicker emission lines in the region of red light, around 650 nm, match up with the thick lines of nitrogen in the same region. The gas contains nitrogen.
Finally we have argon, which fills in the last bits of lines around the regions of purple, yellow, and red light. Most of the emission spectrum appears to be coming from this, so the gas contains argon.
The answer is every gas except for hydrogen. This means the correct answer is choice E, helium, oxygen, nitrogen, and argon.
Let’s summarize what we have learned in this explainer.
- Every atomic element has a unique absorption and emission spectrum.
- Absorption spectra are lit with dark bands; emission spectra are dark with lit bands.
- The wavelength of the photon for a band on a spectrum depends on the energy difference between the electron energy levels.