![]() If the two slides are rotated by an angle in between these two extremes, they will form an ‘X’ pattern as seen on the left. If they are at right angles, you get the middle pattern. It is just like a single grating, except that they are probably slightly out of alignment because the film in the frame can shift slightly. If the two gratings’ slits are in the same direction, it looks like the pattern on the left. There are a variety of possibilities depending on how the two gratings are aligned. Next, I ask the students to predict what they will see if the light passes through two gratings laid on top of each other. This asymmetry of the pattern above can be understood by looking at the side view diagrams below. They also find that if the grating is tilted about a horizontal axis, the pattern becomes asymmetric, like the diagram below. They find that rotating the grating about a vertical axis, causes the line of dots to rotate by the same amount. They might also notice a fainter pair further out if the classroom is dark. They notice that the pattern consists of a very bright spot with two bright spots on either side. I give the students some time to do open exploration with a single grating. You can find more information about laser safety here and here. You should also let them know that the green, blue and UV laser pointers are a real danger. However, it is always a good idea to practice safe techniques. If the students are using red laser pointers this safety procedure isn’t needed. Keeping the beams vertical helps ensure that no stray laser beams go into anyone’s eyes. The students should hold the gratings horizontally and direct the beam down through them onto the table. ![]() These gratings can be purchased here for about a dollar each. I use gratings with 5360 lines per inch because they provide a large angular spread, which allows students to examine the interference patterns in small groups at their desks. Qualitative Observations of Interference Patternsīefore looking at Franklin’s work, the students need a firm grounding in the interference patterns of diffraction gratings which are much simpler. I found a couple of short videos and a really simple, cheap demo that shows why the interference pattern formed by DNA provides clear evidence of its helical structure! Her work on the X-ray crystallography of DNA fits perfectly into a lesson about the interference of light! As well as showcasing an important female scientist, an examination of her work can deepen students’ understanding of interference patterns and it highlights a very important connection between biology, chemistry and physics. I started to compile a data base of good examples when I found myself completely distracted by the work of Rosalind Franklin. (If you would like to be a part of this - please send me an email!) Sara Cormier (Physics Instructor at McMaster University) and I are trying to develop resources that will help teachers to do this. Roberta Tevlin, Editor OAPT Newsletter, Teacher at Danforth need to incorporate more diverse examples of scientists in our courses. ![]()
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