1. The problem statement, all variables and given/known data A laser beam of diameter = 3.1mm in air has an incident angle = 26 degrees at a flat air-glass surface.If the index of refraction of the glass is n=1.5, determine the diameter of the beam after it enters the glass. 2. Relevant equations n1sinθ1=n2sinθ2 3. The attempt at a solution okay, well the initial application of snell's law is obvious. The laser is going from air, where n = 1 into glass where n = 1.5 So, (1)sin(26) = (1.5)sinθ2 θ2 = (approx) 17 degrees or 16.99 I know how to find the diameter of the laser, the solutions have this equation: d1/cosθ1 = d2/cosθ2 and simple plug and chug would yield the answer 3.3 mm But I just don't understand where this second equation came from. Can some one walk me through the sense behind it? My book doesn't even mention it, so perhaps its obvious (just not to me).