  The opacity of graphene is known to be approximately given by the
fine-structure constant $\alpha$ times $\pi$. We point out the fact that the
opacity is roughly independent of the frequency and polarization of the light
can be attributed to the topological charge of the Dirac points. As a result,
one can literally see the topological charge by naked eyes from the opacity of
graphene, and moreover it implies that the fine-structure constant is
topologically protected. A similar analysis suggests that 3D topological
insulator thin films of any thickness also have opacity $\pi\alpha$ in the
infrared region owing to the topological surface states, indicating that one
can see the surface states by naked eyes through an infrared lens. For 3D Dirac
or Weyl semimetals, the optical absorption power is linear to the frequency in
the infrared region, with a linearity given by the fine-structure constant and
the topological charge of Weyl points.
