Diffraction of Light: A Laboratory Experiment in General Physics II, Lab Reports of Physics

Download Diffraction of Light: A Laboratory Experiment in General Physics II and more Lab Reports Physics in PDF only on Docsity! Phy 212: General Physics II page 1 of 5 Instructor: Tony Zable Experiment: Diffraction & Wave Properties of Light Objectives:  To observe the diffraction behavior of light  To determine the spacing distance for a diffraction grating and a CD  To determine the wavelengths of the yellow spectral emission line produced by the mercury atom Introduction: The diffraction of classical waves refers to the phenomenon wherein the waves encounter an obstacle that fragments the wave into components that interfere with one another. Interference simply means that the wave fronts add together to make a new wave which can be significantly different than the original wave. For example, a pair of sine waves having the same amplitude, but being 180o out of phase will sum to zero, since everywhere one is positive, the other is negative by an equal amount. A diffraction grating is a transparent material into which a very large number of uniformly spaced wires have been embedded. One section of such a grating is shown in Figure 1. As light passes through the grating, the light waves that fall between the wires undergo diffraction propagate straight on through. The light that impinges on the wires, however, is absorbed or reflected backward. At certain points in the forward direction the light passing through the spaces (or slits) in between the wires will be in phase, and will constructively interfere. The condition for constructive interference can be understood by studying figure 1. Whenever the difference in path length between the light passing through different slits is an integral number of wavelengths of the incident light, the light from each of these slits will be in phase, and will form an image at the specified location. Mathematically, the equation that describes angular position of diffraction maxima for a grating is simple and reminiscent of 2-slit interference: d.sin  = m where d is the distance between adjacent slits (which is the same as the distance between adjacent wires),  is the angle the re-created image makes with the normal to the grating surface,  is the wavelength of the light, and m = 0, 1, 2, . . . is an integer. Diffraction gratings can be used to split light into its constituent wavelengths (colors). In general, it gives better wavelength separation than does a prism, although the output light intensity is usually much smaller. By shining a light beam into a grating whose spacing, d, is known, and measuring the angle, , for the resulting diffraction pattern (maxima), the wavelength, , can be determined. This is the manner in which the atomic spectra of various elements were first measured. Alternatively, one can shine a light of known wavelength on a regular grid of slits,  d.sin   d Figure 1: The geometry of the diffraction grating Phy 212: General Physics II page 2 of 5 Instructor: Tony Zable and measure their spacing. You can use this technique to measure the distance between grooves on a CD or the average spacing between the feathers on a bird’s wing. Part 1: Diffraction of laser light using a pin needle 1) Place a piece of tape over the on/off button of a laser pointer then place it on its side on the table top. 2) Aim the laser pointer toward a wall on the far side of the lab room. 3) Observe the pattern of the beam on the wall. Record your observations and sketch the beam pattern. 4) Place a pin needle (vertical orientation) about 5 to 8 cm in front of the laser pointer, directly in the path of the light beam (see Figure 2A). 5) Observe the pattern of the beam on the wall (make sure that the pin needle is directly in the light path). Record our observations and sketch the beam pattern. 6) Remove the pin needle and position it such that the head of the needle is directly in front of the beam path (see Figure 2B). 7) Observe the pattern of the beam on the wall (make sure that the head of the pin needle is directly in the light path). Record your observations and sketch the beam pattern. 8) How do the patterns in steps 5 and 7 compare? Laser Pointer Laser Pointer Pin Needle Pin Needle Light beam Light beam Head Light Pattern on the wall Light Pattern on the wall Figure 2A: Figure 2B: