Light is incident on a glass-air interface from the glass side

Question

(n=1.5), and researchers want to use the evanescent field on the air side (n=1.0) to excite the
molecules adhered to the surface. It is desired that the evanescent field extend a distance δ = 2 µm
into the air side when using light of free-space wavelength of 1 µm.
a. How close to critical angle must the incident beam be? (Give the difference ∆θ = θ1 - θc)
b. Considering that a beam with finite width contains rays with a spread of angles due to diffraction,
how wide must the beam be so that the angular spread is just equal to the difference in angle
found in part a?