  The magnetic-field dependence of optical reflectivity [$R(\omega)$] and
optical conductivity [$\sigma(\omega)$] spectra of the ideal type-I Weyl
semimetal TaAs has been investigated at the temperature of 10 K in the
terahertz (THz) and infrared (IR) regions. The obtained $\sigma(\omega)$
spectrum in the THz region of $\hbar\omega\leq15$ meV is strongly affected by
the applied magnetic field ($B$): The Drude spectral weight is rapidly
suppressed and an energy gap originating from the optical transition in the
lowest Landau levels appears with a gap size that increases in proportion to
$\sqrt{B}$, which suggests linear band dispersions. The obtained THz
$\sigma(\omega)$ spectra could be scaled not only in the energy scale by
$\sqrt{B}$ but also in the intensity by $1/\sqrt{B}$ as predicted
theoretically. In the IR region for $\hbar\omega\geq17$ meV, on the other hand,
the observed $R(\omega)$ peaks originating from the optical transitions in
higher Landau levels are proportional to linear-$B$ suggesting parabolic bands.
The different band dispersions originate from the crossover from the Dirac to
the free-electron bands.
