Spectroscopic is the adjective for anything to do with spectroscopy — the study of light split into its colours. It describes methods, instruments, evidence, and results: spectroscopic analysis, spectroscopic data, a spectroscopic study of a star, spectroscopic evidence that water exists on a planet. In plain terms, if something is 'spectroscopic', it was found out by looking at which colours of light a thing gives off or absorbs. Note that this is NOT the instrument itself — the instrument is a spectroscope. A dictionary line saying 'an instrument used for forming and looking at spectra' has slipped; that definition belongs to spectroscope. Spectroscopic is purely a describing word, and you will nearly always see it in front of a noun, never standing alone after 'is'. Scientists write 'spectroscopic measurements were taken'; nobody says 'the result was spectroscopic'. This is specialist vocabulary — physics, chemistry, astronomy, materials science, and the papers written in those fields. An A1-A2 student will not need to produce it, but may well meet it while reading about space or science, and it helps to know the word simply flags 'done by studying light'.