Why Diffraction and not Refraction Spectroscope?
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.
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
Illinois Central College
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Update: June 2012