Brewster's law is - 

(where μ = refractive index and  = polarising angle) 

Concept:

• The angle of incidence at which a beam of un-polarized light falling on a transparent surface is reflected as a beam of completely plane polarised light is called polarising or Brewster angle. It is denoted by ip.
• Brewster's law: It states that when a ray is passed through some transparent medium having refractive index μ at any particular angle of incidence, reflected ray is completely polarized; and the angle between reflected and refracted ray is 900.

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μ = tan θB = tan ip

Where μ = refractive index and θB is Brewster's angle or polarizing angle (ip).

Explanation:

• From above it is clear that the Brewster law states the relationship between light waves and polarized light. 
• The correct option is μ = tan ip