(a) a is the lower slope line
(b) From Snells law, the slope of this graph is nglass = 1.5
(c) From nglass sin θc = nair sin θair … we want to use the point where sin θair = sin 90 = 1. This point will correspond to the related angle in the glass and since it’s the critical point, this will allow you to find θc
So we find sin θa = 1 from the graph and find the corresponding value of sin θglass that goes with it.
Extending the lower slope line up to 1.0, we get a sin θg value and then set that value = sin θg and solve for θ which is the critical angle.
(d)