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Here below the 3 spectrum currently used in astrophysics and illustrating Kirchhoff's laws (emission, absorption and continuous spectrums),:

spectrums

I just want to know : what's the utility of the optical slit on the left for each case (having a plane wave ?) ?

Regards

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The purpose of the slit is to prevent overlap of different spectral components at the screen or detector.

Imagine what happens if the light contains just one spectral component and there is no slit: there would be a spot of light on the screen, as wide as the face of the prism. Now add another spectral component with a slightly different wavelength, and there would be two wide stripes, overlapping. It would not be possible to distinguish the two wavelengths by examining the light distribution on the screen. When a slit is included, the stripes corresponding to the spectral components are narrowed in proportion to the slit width, so if the slit is sufficiently narrow it is possible to see two stripes, each corresponding to one of the wavelenghs.

The minimum wavelength difference a spectrometer like this can distinguish is proportional to the slit width.

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This is a simplistic representation of a spectrometer. The slit is just used to shape the light into a nice rectangular beam that fits the prism height and keeps the beam narrow so a good image is produced. In real spectrometers slits, tubes, fibres and lenses are used to get the best beam shape onto a diffraction grating (prisms not common?). The incoming light is never a perfect rectangular beam, it takes a lot of design and forethought to design a good spectrometer based on the nature/shape/intensity of the light source.

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  • $\begingroup$ -@PhysicsDave thanks, just a last precision, the slit must not be too narrow in order to avoid diffraction effects, has it ? $\endgroup$ – youpilat13 Aug 11 '18 at 13:05
  • $\begingroup$ If the slit is too narrow the biggest problem would likely be intensity. But the slit in your diagram is just for shape, the prism is doing all the work due to refraction. There are many types of spectrometers and maybe in some diffraction from the aperture/slit might be something they worry about. $\endgroup$ – PhysicsDave Aug 11 '18 at 13:19

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