Here's how to show the discrete atomic spectra to students.
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Teachable Topics:
- Emission and absorption
- Emission and absorption in the interstellar medium
- Diffraction gratings
Spectroscopy is a method that allows us to determine the chemical composition of many things in the universe, namely stars, clouds, and other phases of the interstellar medium. Spectroscopy is made possible because of the behavior of electrons in the atoms that make up these bodies.
An atom in its ground-state configuration consists of the nucleus and a cloud of electrons, each of which are in their lowest energy level. The electrons are likely to stay in their lowest energy state. However, if the atom is disturbed by any mechanism that gives it energy (eg. a collision with another atom, a photon, or an electron), the electrons can jump from their ground state level to a level of higher energy within the atom, causing the atom to become "excited". Conversely, once an electron is in a higher energy level (ie. the atom is excited), the electron can jump back down to a lower state by getting rid of the extra energy in the form of a photon. Every element in the periodic table has different spacings betweent the possible energy levels to be populated and therefore each element has a unique spectrum associated with it.
The amount of energy required to jump up or down to the various energy levels in an atom is discrete. In other words, only a very specific amount of energy will allow an electron to jump to a certain level. If too much or too little energy is given to the electron, the transition will not occur. Similarly, when an electron leaves a higher energy level to occupy a lower one, the amount of energy in the photon released for this transition is always the same for a specific element. Each of these specific energies has a specific corresponding wavelength given by Planck's Law:
E = hν
E = h(c/λ)
so λ = hc / E
These specific wavelengths can be detected. Using a diffraction grating, the various wavelengths in the visible portion of the spectrum can be viewed.
Apparatus:
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Diffraction Grating
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Claw clamp and optical bench stand
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Emission tubes
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Emission tube lamp
Procedure:
1. Plug in the emission tube lamp.
2. Place the diffraction grating into the clamp and the clamp in the stand infront of the emission lamp. You can also view the spectra through the diffraction grating by holding the grating in your hand at a sufficient distance from the emission tube.
3. Put one emission tube into the lamp and turn the lamp on.
4. View the emission spectrum through the diffraction grating.
SAFETY WARNINGS!
The emission tubes can become hot very quickly so it is best to only leave the lamp on while viewing the spectra. Use a paper towel or glove to remove the tubes afterwards.
