The<unk>  (or<unk>  of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium.  For example, typical glass has a<unk>  of 1.5, which means that light travels at 
1
/
1.5
=
0.67
{\displaystyle 1/1.5=0.67}
 times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their<unk> . First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a<unk>  different from that of their surroundings.
Refractive index
Intraocular pressure
Visual acuity
Fundal height
Blood pressure
Vitreous humour
Aqueous humour
Blood sugar