refractive index 
noun
The ratio of the velocity of light in a vacuum to its velocity in a specified medium.Light at each end of the optical spectrum is dispersed by a different amount, since the refractive index of any medium depends on frequency....
- Methods include visual examination, study of the absorption spectrum, and measurement of refractive indices, specific gravity, and thermal conductivities.
- This condition is fulfilled if the refractive index of the object is larger than the refractive index of the surrounding medium and if the beam sizes are larger than the size of the trapped object.
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