The critical angle for a light ray travelling from denser to rarer medium is defined as the angle of incidence at which the light refracts at 90∘ (angle of refraction).
If a ray of light travels from water (μw) to air (μa).
Find the critical angle?
sin-1 (μa/μw)
Apply Snell's law at interface
μw.sin(ic)=μasin(90∘),
where sin(90∘)=1
ic=sin−1μaμw