Dense plasmas with uniform and constant density and temperature are generated by passage of a planar shock wave through argon gas. Our experiments in argon have produced plasmas with a temperature of 1.4—2.2eV and a density of 0.0083—0.015gcm3. The opacity of the plasma is measured versus wavelength by recording the rise time of emitted light. Measurements in visible light are compared with the calculations using the average atom model, and the agreement is well. In data treatments, the opacity and the reflectance R(λ) at the baseplate / argon interface are simultaneously worked out. The resultant R(λ) in this work are all around 0.4, which is in agreement with that given by Erskine.