In this experiment, you look at the colors of the light that the atoms of a particular element give off when excited by electricity. This is the same type of data that led some of the greatest scientific minds of the twentieth century to develop the concept of the atom. The patterns of those colors give us insight into the mysteries of the structure of the atom. Like the New Jersey Turnpike, the electrons in the various energy levels of the hydrogen atom can exit only in certain specific ways.
What You Need
- diffraction grating
- tube of hydrogen
- high-voltage power supply to excite the hydrogen
- Insert the hydrogen tube into the high-voltage power supply. Make sure the good electrical contact is established between the electrodes of the hydrogen tube and the power supply.
- CAUTION: Do not touch the electrical contacts of the high-voltage power supply once it is activated. Follow all manufacturer's instructions for safe use of this equipment.
- Darken the room.
- Hold a diffraction grating with the scribed lines parallel to the tube in front of your eyes, as indicated in Figure 120-1.
- Observe the image of the glowing hydrogen tube broken down by the diffraction grating. If you have a spectrometer, observe the light from the hydrogen tube and identify the positions of each of the lines you see. Look for the transmitted light to the left and right of the central image from which the glowing hydrogen tube is located. You may need to use your peripheral vision to see the entire effect.
The light transmitted through the diffraction grating is not a continuous rainbow.
The light is broken down into a few bright vertical lines.
The details of the lines you see are summarized in Table 120-1.
Why It Works
On the New Jersey Turnpike, if you get on at Exit 6 and go to Exit 7, you pay a $0.80 toll. If you go from Exit 6 to Exit 8, you pay $1.20. In a hydrogen atom, if an electron goes from the third energy level to the second energy level, only red photons (with a wavelength of 656.3 nanometers) are released. But, if an electron goes from the fourth energy level to the second energy level, only blue-green photons (with a wavelength of 486.1 nm) are emitted.
On the New Jersey Turnpike, nothing is between Exit 6 and 7, and you never have to pay a toll between $1.20 and 0.80. The hydrogen atom does not produce a photon whose color is between red and blue-green.
Einstein's interpretation of the photoelectric effect leads us to the conclusion that photons have a certain specific energy based on their frequency. Niels Bohr developed a model of the hydrogen atom based on the idea that the electrons are found in certain specific energy levels, but not in between. A particular change in energy levels results in a photon of a particular color.
When viewed through a diffraction grating, the light from the excited hydrogen atoms does not result in a full rainbow. It, instead, produces only specific brightly colored lines, corresponding to specific wavelengths. Each wavelength is associated with a change from one energy level to another. The bigger the jump, the shorter the wavelength.
Other Things to Try
The spectral breakdown of light emitted by a hydrogen atom can also be detected using a highsensitivity light sensor, such as PASCO part number PS-2176. The result for this is shown in Figure 120-2.
Observation of separate colors from glowing hydrogen gas confirms the model of the atom developed by Niels Bohr, in which electrons occupy specific energy levels. Because electrons cannot be between the established energy levels, many colors (or photon frequencies) are not produce