If light of wavelength of 750 nm is incident at interface of a medium of refractive index of n_1= 1.5 and another medium of refractive index of n_2= 1.Calculate the angle at which the penetration depth of evanescent field in 2nd medium will be minimum? (in deg.)
41.8
61.8
43
45
Answers
Answer:
Total internal reflection means that light is fully reflected at the interface between two transparent media if the angle of incidence (i.e., the angular deviation from perpendicular incidence) is larger than the so-called critical angle. That critical angle is determined by the equation
critical angle for total internal reflection
where n1 is the refractive indices of the medium from which the beam comes, and n2 the refractive index of the other medium. Beyond that angle, Snell's law for the calculation of the output angle could not be fulfilled for any real output angle. Essentially, the component of the wave vector along the interface, which would have to be identical for the incoming and the transmitted beam, is so large for the incoming beam that it cannot be matched by a transmitted beam even with an output angle of 90°.