Under which condition will light undergo total internal reflection?

Under which condition will light undergo total internal reflection?

• A. The angle of incidence is less than the critical angle
• B. The angle of incidence equals the critical angle
• C. The light is moving from a less dense to a more dense medium
• D. The angle of incidence is greater than the critical angle

Answer: (D)

Total internal reflection happens when light moves from a denser medium to a less dense medium and strikes the boundary at an angle larger than the critical angle. The critical angle is the angle of incidence for which the refracted ray travels along the boundary (sin theta_c = n2/n1, with n1 > n2). If the incidence angle exceeds theta_c, Snell’s law would require a refracted ray with an angle greater than 90 degrees, which isn’t possible, so all the light stays inside the original medium and is reflected back. That’s why the condition described—an angle of incidence greater than the critical angle—produces total internal reflection. If the angle is smaller than theta_c, some light refracts into the second medium; if it’s exactly theta_c, the refracted ray glides along the boundary; and total internal reflection cannot occur when light moves from a less dense to a more dense medium.

Total internal reflection happens when light moves from a denser medium to a less dense medium and strikes the boundary at an angle larger than the critical angle. The critical angle is the angle of incidence for which the refracted ray travels along the boundary (sin theta_c = n2/n1, with n1 > n2). If the incidence angle exceeds theta_c, Snell’s law would require a refracted ray with an angle greater than 90 degrees, which isn’t possible, so all the light stays inside the original medium and is reflected back.

That’s why the condition described—an angle of incidence greater than the critical angle—produces total internal reflection. If the angle is smaller than theta_c, some light refracts into the second medium; if it’s exactly theta_c, the refracted ray glides along the boundary; and total internal reflection cannot occur when light moves from a less dense to a more dense medium.