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Name: Jeff
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Question:
Why is it called a diffraction grating spectroscope? The diagrams of how it works seem to show light coming in the slit, then refracting (NOT diffracting) and reflecting through the bumpy non-air medium material, then projecting the separated colors against the wall. I thought diffraction was when a wave goes around a edge/corner but all the while staying in the same medium.



Replies:
Jeff,

If you want to use the correct term for the pattern produced by the grating, the word is interference. Refraction is due to a "change of speed" of the light as it passes through a material. A diffraction grating has many parallel slits of very tiny width. Light through adjacent slits experiences interference just as occurs for light passing through two parallel slits. Light diffracts as it leaves each slit. The diffracted light from each slit crosses paths with diffracted light from adjacent slits. This results in interference. For very narrow slits the diffraction is broad enough that only interference effects are seen. If slit widths are large enough, the diffraction pattern and interference pattern will "overlap". This will sometimes result in a few lines of the interference pattern missing. A diffraction grating's slits are so narrow that such an effect is not observed. Only the innermost portion of the interference pattern is seen.

As for what the light actually does within the diffraction grating, or within parallel slits, that is a matter of interpretation and opinion. There are various models that are all used because they all work. Some involve quantum physics. Some involve each point of light generating light in all forward directions with interference from neighboring light eliminating all but straight-line light until the slit edges remove the neighbors. Remember that a great deal of science is used because it provides correct and useful predictions rather than because we know exactly what is happening.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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