  Twisted bilayer graphene (TBG) represents a highly tunable, strongly
correlated electron system owed to its unique flat electronic bands. However,
understanding the single-particle band structure alone has been challenging due
to complex lattice reconstruction effects and a lack of spectroscopic
measurements over a broad energy range. Here, we probe the band structure of
TBG around the magic angle using infrared spectroscopy. Our measurements reveal
spectral features originating from interband transitions whose energies are
uniquely defined by the twist angle. By combining with quantum transport, we
connect spectral features over a broad energy range (10 to 700 meV) spanning
several superlattice minibands and track their evolution with twist angle. We
compare our data with calculations of the band structures obtained via the
continuum model and find good agreement only when considering a variation of
interlayer/intralayer tunnelling parameters with the twist angle. Our analysis
suggests that the magic angle also shifts due to lattice relaxation, and is
better defined for a wide angular range from 0.9{\deg} to 1.1{\deg}. Our work
provides spectroscopic insights into TBG's band structure and offers an optical
fingerprint of the magic angle for screening heterostructures before
nanofabrication.
