The second source of radiation is atomic radiation. The atom consists of protons and neutrons in the nucleus. The protons are positively charged. In orbit around the nucleus are negatively charged electrons. See the diagram below. These electrons are restricted to very specific orbits which have associated with them very specific energies. When an electron moves from one orbit to a different orbit there is a change in the amount of energy associated with the electron. If the electron moves from an orbit with high energy (for example, the orbit labeled 3 below) to an orbit with low energy (the orbit labeled 1), then the energy given off is released in the form of radiation. This is the radiation we see with atomic light. A mercury vapor lamp is an example of this type of radiation.

There are a few points which should be made about atomic radiation. When the electron jumps from the second energy level to the first energy level it gives off a specific photon. If the electron should jump from the third energy level to the first energy level it gives off a different photon. Since the energy change is larger from the third level to the first, the amount of energy in the photon is also larger. A larger energy photon has a higher frequency and shorter wavelength. The energy change from level 3 to level 1 is always the same for the same type of atom. Therefore the photons of light will also be identical. It is important to note, however, that the photons from a different atom are very different. The light which is characteristic of hydrogen is never the same as the light which is characteristic of helium. Therefore, when astronomers identify light coming from a star they are always able to identify the atom which is the source of that light. Identification of this light is very important in identifying the atoms within a star.

In order for the electron to move from a lower energy state to a higher energy state the electron must gain energy. There are many ways in which the electron can gain this energy. One way might be electrical energy. This is the method used in fluorescent lamps. Another way is for the electron to absorb the energy of a photon. However, the energy of the photon must be exactly the same as the energy change from the lower orbit to the higher orbit. Therefore if the energy change from level one to level two corresponds to blue light, red light will not allow the transition to take place. In fact, a different energy blue photon will also not allow the transition to take place. The energy and therefore frequency of the photon must be exact.

When an electron gives off a photon in a transition from a higher energy to a lower energy we call this process emission. Conversely, if the electron gains energy from a photon and makes the transition from a lower energy level to a higher energy level, we call this process absorption.

You have certainly seen the colors of a rainbow or the way light splits apart into its colors as it passes through crystal or glass. When white light splits up into all of its component colors we see what is referred to as a continuous spectrum.

If we were to view the light from a mercury vapor lamp we would see only those very specific photons associated with the many transitions of electrons in mercury from higher states to lower states. Since only specific colors, not all colors, would be seen in this spectra, it is not continuous. The colors we do see are due to the emission of photons from mercury. Therefore we call this an emission spectra. Emission spectra are characteristic of hot thin gases.

If the light from a continuous spectrum were to pass through a cool thin gas, the electrons of that gas could absorb some of the photons from the continuous spectrum. If the spectrum were now viewed, certain colors from the spectrum would be missing due to their absorption by the atoms in the gas. This type of spectrum is referred to as an absorption spectrum.

Astronomers will use these various spectra to identify the atoms in space and in the bodies of stars. More discussion on the spectra will occur later in the course.

If an ambulance or police car with the sirens on was to pass you, you would notice a change in the frequency or pitch of the siren. The pitch of the siren would be higher while the siren was approaching you and lower while the siren was moving away from you. A similar occurrence may take place while at a railroad crossing if the train is sounding its whistle. This change in the frequency of the wave is called the Doppler effect. A similar change may take place for light waves as well as sound waves. By measuring the Doppler effect, or the change in frequency of the wave (light or sound) it is possible to determine the speed of the source. This is the principle behind radar guns used by police officers. A similar technique is used in astronomy to measure the speed at which galaxies or stars are moving towards us or away from us. It as also been used to measure the movement and rotation of planets. The Doppler effect can only measure speed towards or away, it cannot measure speeds perpendicular to the line of sight of the observer.

The applet for the Doppler effect shows the wave fronts in front and behind a moving source. Click on the link and then click on the "Doppler" link in the left hand frame. You do not need a 4.0 level browser to view this applet. Vary the speeds and see how the ripples change.

This page was last updated on 06/13/01.