Let's first have a look at the visible colour spectrum. The typical human eye can perceive radiation in the
range 390 - 750 nm. Here is a simple depiction of this range: (Source: Wikipedia commons, public domain)
Note however that the human eye does not "see" these colours as separate wavelengths. "Colour" is constructed by
the brain from colour receptors in the human eye, with peaks at (Short = "Blue") 420-440 nm, (Medium = "Green")
534-555 nm, and (Long = "Orange/Red") 564-580 nm.
We will come back to this later...
Next: Making a simple spectroscope