Total internal reflection occurs under which condition?

• A. It occurs for any incident angle
• B. It occurs only when moving from a rarer to a denser medium
• C. It occurs when traveling from slower to faster, with large incidence angle
• D. It occurs when traveling from slower to faster, with angle above the critical angle

Answer: (D)

Total internal reflection happens when light moves from a medium with a higher refractive index to a medium with a lower refractive index, and the angle of incidence is large enough that refraction would require a transmitted ray with sin θ2 greater than 1. In that case, no refracted wave exists in the second medium and all the light is reflected back into the first medium. The critical angle θc is defined by sin θc = n2/n1 (with n1 > n2). If the incident angle θ1 exceeds θc, total internal reflection occurs.

So the described condition—moving from a slower (denser) medium to a faster (less dense) one and having the incident angle exceed the critical angle—matches when total internal reflection happens. The other scenarios don’t satisfy both the direction (dense to rare) and the requirement θ1 > θc for reflection to be total.