If light goes from water to air at incidence 60°, will total internal reflection occur?

• A. No
• B. Only for incidence less than the critical angle
• C. Only for incidence 0°
• D. Yes

Answer: (D)

Total internal reflection happens when light tries to go from a medium with a higher refractive index to one with a lower refractive index and the angle of incidence is larger than the critical angle for that pair of media. The critical angle is set by sin(theta_c) = n2/n1, where n1 is water (1.33) and n2 is air (1.00). That gives theta_c ≈ arcsin(1/1.33) ≈ 48.8 degrees. Since 60° is greater than the critical angle, the refracted ray cannot exist; all the light stays inside the water and is reflected back. So total internal reflection occurs.