In the Classroom
tested demonstrations
Using Lasers To Demonstrate Refraction, Diffraction, and Dispersion Submitted by:
Elvin Hughes, Jr., and L. H. Holmes, Jr. Department of Chemistry and Physics, Southeastern Louisiana University, Hammond, LA 70402
Checked by:
Arnold George Department of Chemistry, Mansfield University, Mansfield, PA 16933
The demonstration of refraction and diffraction in the classroom can be accomplished relatively easily using lasers. A single laser can be used to show simple refraction and, qualitatively, the concept of refractive index. The laser is set up on a desk so that it shines on the wall of the classroom and the spot made by the light dot is marked. Then a liquid in an Erlenmeyer (conical) flask is placed in the beam (Fig. 1). Refraction of the beam will change the position of the light dot. A different liquid will give a different position of the light dot, qualitatively illustrating refractive index. One can also use prisms made of different materials to illustrate this, but common liquids such as water and alcohol seem to make a stronger impression on students. Diffraction can be illustrated by placing a diffraction (transmission) grating in the beam of the laser. The various order diffraction beams can be shown on the wall of the classroom. The distance from the grating to the wall (H in Fig. 2) and the distances from the zero order to the first, second, and higher order beams (L1 and L2 in Fig. 2) can be measured. Using the Bragg equation, nλ = 2d sin θ, one can then either calculate the wavelength of the laser from the line spacing of the grating or vice versa. Two lasers arranged in the configuration in Figures 1 and 2 allow the same measurements as above, but two different wavelengths of light can be used. With the prism or
the liquid in a conical flask (Fig. 1), this arrangement illustrates dispersion as well as refraction and refractive index. The dependence of refractive index on wavelength is also readily shown, since the red and green light beams will be refracted by different amounts. With a transmission grating and the setup in Figure 2 one can demonstrate the basic phenomenon of diffraction and illustrate the Bragg equation. Also, one can calculate the wavelength of one laser from the wavelength of the other using the Bragg equation. Or the line spacing on the grating can be calculated from the wavelength of either laser. Comparison of the observations using prisms and gratings also readily shows a difference in the way prisms and transmission gratings “disperse” light. With the prism, the green light is bent more than the red light in accordance with what is normally called dispersion. With the transmission grating, the red light in each of the beams of order 1 or greater will be displaced more than the green light. This is consistent with the Bragg equation, since the angle of diffraction (or “dispersion by diffraction”) is directly proportional to the wavelength.
Figure 1. Refraction and dispersion with a prism or liquid.
Figure 2. Diffraction and “dispersion” with a transmission grating.
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Journal of Chemical Education • Vol. 74 No. 3 March 1997
