Have you ever wondered why shining light on a glass of water causes rainbows to appear? Or noticed the colors that reflect from a CD or DVD? Light is an electromagnetic wave, and visible light is the small range of electromagnetic waves we are able to see. White light contains all the colors of the visible spectrum (the famous ROY G. BIV), and these colors can be dispersed, or separated, in various ways. Perhaps the most familiar is the use of transparent objects like a glass prism. When light moves from one medium to another and changes speed, the light bends, but different wavelengths of light bend different amounts, “splitting” the light beam into its constituent colors. A different way to disperse light uses something called a diffraction grating, which is a finely ruled piece of plastic or glass. Although the mechanism is a bit more complicated, passing the light through the diffraction grating changes the direction of the light, and different wavelengths bend different amounts.
A tool called a spectrometer uses prisms or diffraction gratings to disperse and collect light in controlled manner, allowing us to see which colors of light are being emitted or reflected from objects and substances. Spectrometers can be expensive, bulky, and complex, but you can use the same principles in your own home or classroom to create cheap, portable, and simple instruments called spectroscopes.
In this activity, you will make a spectroscope that can separate light into its hidden components, and you will use the spectroscope to understand why light sources and colored objects appear the way they do.
- Paper towel tube
- Aluminum foil (1 foot)
- Masking tape
- Diffraction grating sheet or slide (single axis)
Cut a 6-inch by 6-inch piece of aluminum foil and fold it into square quarters.Cut 1 cm of foil diagonally from the corner where all the folds meet. Unfold the sheet and you should have a square hole in the center of the sheet with approximately 1 inch edges.
Tape the edges of diffraction grating over the hole you’ve made. You should be able to see through the grating and the hole.
Center the square hole over an open end of the paper tube and wrap the foil around the tube. Tape this foil wrapping into place.
On the second foil opening, tape two parallel strips of foil over the hole to form a slit. The width of the slit should only be about as wide as a coin.
Wrap this foil over the other end of the tube, centering the slit over the opening. Don’t tape the slit foil. This will allow you to rotate the slit as needed.
Put the diffraction grating end of the tube close to your eye and point the opening of the tube toward an object or a source of light. You should see a colored pattern on the walls of the tube. Attempt to dim all other light sources or view objects in a dark room. Never look directly at the sun!
Diffraction gratings disperse light into its individual components, which appear as spectral lines. The narrow opening of the spectroscope helps increase the resolution of an image and decreases the chance that light from other sources will enter the spectroscope.
White light contains wavelengths of light from the entire visible spectrum, so it will appear as a uniform rainbow band, with no gaps between the lines. Other light sources have different constituent wavelengths, producing a specific pattern of discrete lines inside the spectroscope that is unique to the substance or light source. Fluorescent bulbs, for example, have an inner coating that absorbs some wavelengths of light and radiates others, so only certain colors will appear in the spectroscopic image.
The pattern of light dispersion is predictable and follows a measurable pattern from light waves with the lowest energy and highest wavelength (red colors) to those with the highest energy and lowest wavelength (violet colors). See This Photon Walks Into a Crystal for more on the relationship between light energy and wavelength, and read here to learn more about how diffraction gratings work.
- The color black won’t reflect light as much as other colors. If the inside of the tube is lined with black construction paper or paint it may be easier to distinguish certain color bands.
- Use a single-axis diffraction grating. This means the lines in the diffraction grating are parallel to one another. Therefore, the direction of the spectroscope opening needs to be adjustable so the image can be aligned with the grating.
- If you want to set up a camera to "see" the light instead of an eye, cut the slit end of the spectroscope at a 45-degree angle. Light will go directly into the camera instead of hitting the walls of the tube.
Learning Activity Ideas
- Compare the spectra from different sources of light. Fluorescent, incandescent, and LED light bulbs provide clear examples that not all light sources emit the same types of light.
- You can also compare the light from various LED bulbs. These light sources have well defined and measurable emission peaks. You will notice that some contain more blue wavelengths, while others contain more red wavelengths. Look at neon signs to find their spectra. Consider borrowing elemental spectrum light bulbs from your local high school or university to see other unique spectra. What do you notice about these spectra that is quite different from the light bulb source?
- Detect light that is reflected from colored objects or transmitted through colored filters. See which spectra appear. This can be a great activity to show that certain colors like magenta are combinations of wavelengths of light, rather than a wavelength of light in its own right.
- For more advanced learners: Calculate the distance certain colors will appear on the inside of the spectroscope tube. This can be done using the diffraction grating equation and the thin lens equation in combination. (Distances may be too small to confirm experimentally.)