Astronomy 1402
Department of Physics and Geology Spectra
Equipment Needed Qty Equipment Needed Qty
Spectroscope 1 Set of Colors 1
Gas Spectral Lamps 13 Ruler 1
Background
When an atom is excited, electrons change their energy levels to higher ones. When electrons from
higher energy levels drop back down to lower energy levels, energy is released in the form of light as
shown in the figure 1. The frequency of the emitted light depends directly on the energy between the
two orbits and this determines the color of the light. Every element has a unique spectrum of light
composed of a set number of different frequencies. If materials are heated, the elemental composition
can be determined by resolving those unique spectrums from the emitted light.
Figure 1. A photon may be emitted when the electron drops from high energy level to low energy level.
White light contains all frequencies of visible light. When white light passes through a prism or a
diffraction grating, its component colors are separated and one can distinguish the colors (like in a
rainbow). A device specifically designed to separate the components of light and atomic spectra is the
spectroscope. A spectroscope contains a narrow slit on one end, a long body, and a diffraction grating at
the other end, as shown in Figure 2. When we look at the spectroscope, we see a strong line from the
light source in the middle. On both sides of the strong line, we will see relatively weaker spectra. The
spectrum is same for both sides but in reverse order. In order to observe the spectrum with your eye,
you may look at the spectrum on one side (left or right side). Figure 3 shows an example of a spectrum
from a Helium gas spectral tube.
Figure 2. Simple diagram of a spectroscope Figure 3. Helium spectrum
