Preliminary results of optical diagnostics of bow shocks in a supersonic wind tunnel by applying dual-hologram shear interferometry technique are discussed. A strong refraction effect of the probing beam penetrating a region in the vicinity of a bow shock over a blunt nose cone model has been discovered. On a signal hologram the effect leads to the disappearance of holographic fringes in a narrow region attached to the shock wave front. A reconstructed interferogram in this region manifests the absence of an interference pattern. Computer simulations were performed for a part of the probing beam penetrating the area of high-density steep gradients of compressed air attached to the central part of the shock front of a bow shock. The compressed area was modeled as a hyperbolic cap. The bow shock was assumed axisymmetric. The simulations made it possible to evaluate angles of deflections and found conformity with reconstructed interferograms (shadowgraphs). It is concluded that in the above-indicated region of bow shocks probing light is deviated refractively into some angles, which could be large enough for light rays to be blocked out and never arrive at the detector (photo film). In the case when interferometric fringes disappear, the effect of strong refraction makes it impossible to measure air density gradients in some critical region.