With a value of 1.33 for water, FTP, what is this index that measures the change of direction in a wavefront when passing from one medium to another?
If it is expressed as a complex number, the imaginary part of this value is an indicator of absorptive properties.  Recent experiments have shown that it can be negative for some materials, which can lead to bizarre effects like reverse Doppler shifts.  Its dependence on wavelength gives rise to chromatic aberration and explains why a prism works.  It is often defined as the ratio of the speed of light in a vacuum to the speed of light in a given medium.  With a value of 1.33 for water, FTP, what is this index that measures the change of direction in a wavefront when passing from one medium to another?
If it is expressed as a complex number, the imaginary part of this value is an indicator of absorptive properties.  
index of refraction (or refractive index)