A block of glass with flat sides is surrounded by air. A ray of light traveling…

A block of glass with flat sides is surrounded by air. A ray of
light traveling in the glass (refractive index
na) is incident on the flat interface with air
at the top surface of the block of glass. You observe that no light
is refracted into the air if the incident ray in the glass has an
angle of incidence (measured relative to the normal to the surface
of the glass) greater than 35.5°. What is the refractive index of
the glass?