There was a question from Jerry in a previous post:
“A simple question for which I can’t find an answer: Why do you have to collimate light spectroscopy? What would happen if you didn’t collimate it?”
The basic idea of spectroscopy is to look at the different colors of light coming from some source. Typically light from a source (like an excited gas) is passed through a diffraction grating that makes different colors bend different amounts (like a prism, only better).
Collimated light is basically light that is all going the same direction. How about I go ahead and draw a picture. This is light from a lightbulb that is collimated then going through a diffraction grating.
Now, what happens if you don’t have the slit to make the light collimated?
Technically, that is still collimated light. However, in this case the problem is that you would get multiple overlapping diffraction patterns. Suppose you have a lightbulb that you want to look at with a diffraction grating. The diffraction grating would create a whole bunch of overlapping lightbulb images of different color. The typical way to fix this is to either use a line-type light source, or pass the light through a slit.
I tried to take a picture of this (a line bulb and a normal bulb), but I don’t think it turned out too well. Well, here is the diffraction image for those two bulbs:
Maybe you can notice a little more detail in the bottom image. Think of this as a whole bunch of different colored vertical filaments that don’t really overlap. Also, maybe you can notice the dark area around green-red for the tube bulb. This might be because they were in tinted glass.