A light ray travels from glass ($\mu = 1.5$) to air. The critical angle for total internal reflection at the glass–air interface is approximately:

**Critical angle formula** (denser → rarer): $\sin C = \dfrac{\mu_{\text{rarer}}}{\mu_{\text{denser}}} = \dfrac{1}{\mu_{\text{denser}}} = \dfrac{1}{1.5} \approx 0.667$. $C = \sin^{-1}(0.667) \approx 41.8° \approx 42°$. **Why option C (60°) is tempting.** Some recall $\sin^{-1}(0.866) = 60°$ — but that's $\mu = 1/0.866 \approx 1.15$, not $1.5$. **A useful sanity check:** as $\mu$ rises, the critical angle drops (denser glass holds light in more aggressively). Diamond's $C \approx 24°$ — that's why it sparkles.