Spectra and the Chemical Make-up of Matter

Spectra can be considered as the fingerprints of matter. Every atom emits its own characteristic spectrum of light. The study of spectra has been an important tool for scientists to identify the chemical composition of substances isolated in chemical, biological and astronomical research. For example, the study of spectra from starlight showed that the sun and some of the stars are made of hydrogen and helium.

Helium was actually discovered in the analysis of solar spectra before it was discovered on earth. In this activity we will learn how scientists were able to discover helium on the sun even though they had not yet isolated it on earth.

During the last century, scientists looked carefully at the spectrum of the sun. They saw something similar to the spectrum that you saw for the incandescent lamp. It was continuous but it had an important difference - dark lines appeared in the sun's continuous spectrum. An example of this type of spectra is represented in Figure 1. The gray area represents light coming through while the black lines show some of the places where no light was seen.

Figure 1: Some of the spectrum of a sample star.

While this spectrum is similar to previous ones we have studied, it also has some unique features. As we understand it we will learn that:

• These dark lines can be used to identify elements as the bright lines in the emission spectra of gases can.
• A process that is similar, but not identical to emission creates the dark regions in this spectrum.
  Begin by comparing the dark lines in the solar spectrum with the line of light from the gases shown in Figure 2.

What similarities do you see between the hydrogen and nitrogen spectra and the spectrum of the star?
Do you think that you see evidence for the presence of hydrogen or nitrogen on the sun? Explain your answer.

We see a rather close match of the energies in the gas spectra with some of the energies of the dark lines of the star's spectrum. These observations are a hint that the process that produced the dark lines is related to the one that produced light in gases.

A sign which photographers sometimes have on darkroom doors reads "Keep the door closed so the dark will not escape." This joke plays on the idea that dark is the absence of light (or energy), so it cannot escape. We will use the same idea with the star's spectrum.

The dark lines are energies that are missing from its spectrum. Each of these energies has been removed while other energies are present.

To understand the processes we need to think about energy in the light. In the previous activity we saw how light is produced by energy changes in atoms. Now think about what needs to happen for one energy of light to be removed from a beam containing all energies of visible light.

Can you use the energy level diagram to describe how a photon of light can be removed from a beam of light?