A ray of light passes from glass into air at an angle of incidence of 40 degrees. The glass has a critical angle of 42 degrees. Which diagram shows what happened to the
ray?

A is correct

Refracted light is almost along the x-axis

Ni sin thetai = Nr sin thetar

In this case thetai is the incident angle and thetar the refracted angle

At the critical angle thetar = 90 deg and

1 = Ni sin thetai   since Nr is the refractive index of air  

Ni = 1 / sin 42 = giving Ni = 1.49 for the glass

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