Brewster's angle for two transparent mediums which have different refractive indexes can be calculated through the formula: Brewster's angle = arctan (n2 / n1), where n1 and n2 are the refractive indexes of the first and second medium. It is calculated by using Snell's law, which is: sin (angle of incidence)/sin (angle of refraction) = n2/n1.

**Find the two refractive indexes**First, find out the refractive indexes of the mediums in question. Each medium through which light can pass through has a different refractive index, which is a dimensionless parameter that indicates how light passes through that medium and determines how much light bends when it enters that medium. Refractive index of for some common mediums are: 1 for air, 1.5 for glass and 1.33 for water.

**Calculate the ratio of the refractive indexes**After finding out the value of the refractive indexes, calculate their ratio. Divide the refractive index of the second medium through which light is to be passed by the refractive index of the initial medium through which light was propagating.

**Calculate Brewster's angle**Calculate the Brewster's angle between the two mediums by taking the arctan of the ratio of the two refractive indexes. For example, if the mediums are air and glass then the ratio between their refractive indexes is 1.5 and the Brewster's angle is approximately 56 degrees.