What is the Minimum Incident Angle to Excite Evanescent Waves in a Glass Prism?

[nmsurobert]

Homework Statement

A glass prism is used to launch an evanescent wave along a glass-air interface. What is the minimum incident angle to excite the evanescent wave?

Homework Equations

The Attempt at a Solution

I don't understand what the question is asking for. I have read and reread the section in my book about evanescent waves it doesn't say anything about this? what does the question mean by "excite"?

 

[gneill]

I believe they are referring to the waves that occur in the close proximity to an interface where total internal reflection is occurring (which should be a strong hint as to what sort of angle you are looking for). One might think that nothing "gets across" the barrier presented by the interface when total internal reflection is occurring, but there are very short range field effects generated there.

 

[nmsurobert]

thats what i was thinking. we've spoken about total internal reflection and its in the context of evanescent waves in the text. thank you!