Light and the Atomic Spectra Lab
White light is really a mixture of all the possible wavelengths of light in the visible spectrum.
We have discussed the difference between a continuous spectrum and a discontinuous spectrum, and you've learned that when all of the light waves in the visible portion of the continuous electromagnetic spectrum are mixed together, your eye sees ‘white’. In this lab, we will show how white light can be spread out into the rainbow of colored light that it is composed of using a prism. We will also show how colors can be ‘stirred’ back together to form white using a color wheel.
Mini-lab One: The Rainbow of Colors in White Light
Turn off the lights in the classroom. Then allow white light from a slide projector to pass through the glass prism and project onto the screen behind. You should see a rainbow of colors appear where the light hits the screen. This is a continuous spectrum, and should convince you that ‘white’ really isn’t a ‘color’, but rather ‘all colors’ mixed together.
Mini-lab Two: Mixing a Rainbow into White
A color wheel (color wheels can be purchased from a school supplier, for instance at http://www.teachersource.com/LightAndColor/PersistenceofVision/MagicColorWheel.aspx). It is possible to make your own. However, if you choose to do this, be sure to get the colors in the correct proportions. If you choose to make your own, it can be spun by fastening it to the end of a drill, and powering up the drill.
Start with the color wheel at rest, noticing how it contains all of the colors in the rainbow. Begin spinning the color wheel, slowly at first, and then, with increasing speed. Notice how, once it is spinning fast enough, the colored wheel turns white. Again, this should convince you that ‘white’ really isn’t a ‘color,’ but rather, all colors at once.
Demonstrations for Chapter 5
Atomic Spectra Viewed Through a Diffraction Grating Demo
The speed of light in a vacuum is . All frequencies of light in a vacuum travel at the same speed. When light passes through media other than a vacuum, such as glass or water, however, all frequencies do not travel at the same speed. In general, light travels at a speed of about in glass and not all frequencies travel at the same speed. Long wavelengths of light, such as red, are slowed down less in glass than short wavelengths like blue. When light traveling in air passes into glass at an angle, its path is bent (diffracted) due to one side of the wave front slowing down sooner than the other side. Since each frequency of light is slowed by a slightly different amount, each frequency of light will bend by a slightly different amount. This allows us to use a prism (a triangular piece of glass) to separate the frequencies of light from each other.
When a beam of white light enters a prism, the red light is slowed and bends slightly as it passes through the boundary between air and glass. The light travels through the prism and is bent again as it leaves the prism. Blue light, which is slowed more when it enters the glass, would also bend upon entering the glass and it bends at a slightly greater angle than red light. The blue light would bend again as it leaves the prism and again would bend slightly more than red light. When the light comes out the other side of the prism, the different frequencies that made up the white light would be traveling at slightly different directions. If the light is projected onto a surface, the observer will see a rainbow of colors as shown above. The rainbow can also be observed with the naked eye looking back into the prism.
Light composed of a mixture of frequencies can also be separated into individual frequencies with a diffraction grating. A diffraction grating is a thin piece of glass or plastic which has thousands of vertical scratches per centimeter on its surface. The method by which a diffraction grating functions is more complex than a prism; it isn’t necessary to examine the method here. When you look through a diffraction grating at a thin beam of light, the light source is still visible, but images of the light source in each frequency present in the original light source will also appear several times beside the original light source.
In the sketches above, the original light source appears in the center and this light source will be the composite color – i.e. the color due to the mixture of the frequencies. To the sides of the original source will be images of the source. There will be an image for each frequency present in the original source. When the images are produced by a diffraction grating, the higher frequencies will appear closest to the original source.
If you look at a source of truly white light, such as a tungsten filament, through a prism or diffraction grating, you will see a complete rainbow of the electromagnetic spectrum. This is because it is heated. If you look at an atomic spectrum (the light emitted by exited atoms of a particular element), you will see only a few lines because the atoms of a particular element emit only a few frequencies of light.
Apparatus and Materials:
- Incandescent light bulb (, tungsten filament) with a plug in socket.
- Spectrum tube power supply
- Several spectrum tubes
- Diffraction gratings (one for each student or pair of students)
Important note: The spectrum tube power supply produces a very high voltage. Never attempt to replace spectrum tubes unless the power supply is turned off. Never touch or even point toward the spectrum tubes unless the power supply has been turned off. It is advisable, that when changing the spectrum tube, you should not only turn the power supply off but also unplug it. This avoids the circumstance of accidentally bumping the on/off switch with your arm while fingers are near the spectrum tubes. The spectrum tube power supply is quite dangerous if not handled properly.
- Pass out diffraction gratings to students and warn them that oily fingerprints on the surface of the grating degrade their function considerably. Effort must be made to handle the gratings only by the frame or mount.
- Use the incandescent bulb first so the students can see the entire white light spectrum. It is best to view the spectra in a darkened room. You can also use this opportunity for the students to properly orient their diffraction gratings. The gratings should be held so that the scratches are vertical.
- Remove the white light and replace it with the spectrum tube power supply. Begin with a simpler spectrum such as hydrogen, helium, or sodium. Insert the spectrum tube into the power supply, keep your hands away from the tube, and turn the power supply on. The spectrum tubes are intended to be turned on for only about at a time. You may turn them on for and then off for and then back on if the students need more time for viewing.
- If there is sufficient light in the room, have the students sketch the spectrum of some of the elements as they view them.
- When you change spectrum tubes, turn off and unplug the power supply. Use hand protection (such as a pot holder) to remove the tube because it will be HOT! After replacing the tube, plug the power supply in and turn it on.
- When you have gone through your supply of spectrum tubes, you may use one of the tubes you used before and see if the students can identify the element from their sketches of the various spectra.
- The demo should be followed by a discussion of how the atoms produce these light spectra.