  We report infrared studies of the Landau level (LL) transitions in single
layer graphene. Our specimens are density tunable and show \textit{in situ}
half-integer quantum Hall plateaus. Infrared transmission is measured in
magnetic fields up to B=18 T at selected LL fillings. Resonances between hole
LLs and electron LLs, as well as resonances between hole and electron LLs are
resolved. Their transition energies are proportional to $\sqrt{B}$ and the
deduced band velocity is $\tilde{c}\approx1.1\times10^6$ m/s. The lack of
precise scaling between different LL transitions indicates considerable
contributions of many-particle effects to the infrared transition energies.
