Light of wavelength 550 nm is incident within glass of refractive indent nG = 1.50 onto a boundary with air at an agle of incidence of 60 degrees; total internal reflection occurs, and an evanescent wave travels a short distance into the air. What is the distance normal to the boundary of which the evanescent wave amplitude decreases by a factor of e?

Light of wavelength 550 nm is incident within glass of refractive indent nG = 1.50 onto a boundary with air at an agle of incidence of 60 degrees; total internal reflection occurs, and an evanescent wave travels a short distance into the air. What is the distance normal to the boundary of which the evanescent wave amplitude decreases by a factor of e?