Grazing Incidence Infrared Reflectivity of La$_{1.85}$Sr$_{0.15}$CuO$_4$   and NbN
  Infrared reflectivity measurements, using p-polarized light at a grazing angle of incidence, show an increased sensitivity to the optical conductivity of highly reflecting superconducting materials. We demonstrate that when this measurement technique is applied to the conventional s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystal, in the superconducting state, we find a reduction of the optical conductivity in the frequency range below 20 meV. The observed frequency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter. 
H. S. Somal, B. J. Feenstra, J. Sch\"utzmann, Jae Hoon Kim, Z. H.   Barber, V. H. M. Duijn, N. T. Hien, A. A. Menovsky, Mari...