Question

Using Snell’s law prove that; n21 x n12 = 1

Answer 1

n12=velocity of light in 1/ velocity of light in 2

n21=velocity of light in 2/ velocity of light in 1

so n21 x n12 = 1

Answer 2

n12 = velocity of light in medium 1/ velocity of light in medium 2

and n21= velocity of light in medium 2/ velocity of light in medium 1 

so by multiplying n12 x n21, all values will be cross cutted and thus 

n12 x n21 = 1