Total Internal Reflection: Practice Problem & Solution

Solution Explained:

To solve this problem, we apply the core principles of Total Internal Reflection. Understanding the underlying formula is key to arriving at the correct answer below:

The light is cut off if the disc covers the area defined by the critical angle. Radius is $r = \frac{h}{\sqrt{\mu^2 - 1}}$. Substituting $h = 4 m$ and $\mu = \frac{5}{3}$, $r = \frac{4}{\sqrt{(\frac{5}{3})^2 - 1}} = \frac{4}{\frac{4}{3}} = 3 m$.