You're absolutely right to be careful with the order of the indices of refraction, as it's a common mistake in problems involving Snell's Law. Let's clarify the details and check your reasoning. ### Snell's Law Snell's Law states: \[ n_1 \sin \theta_1 = n_2 \sin \theta_2 \] If you are going from medium 1 (with index \( n_1 \)) into medium 2 (with index \( n_2 \)), the angles \( \theta_1, \theta_2 \) are measured from the normal in each medium. ### Your Calculation You mention a calculation involving \( 1/1.5 \) when you think it should be \( 1.5/1 \). - If light goes **from air (\( n_1 = 1 \)) into glass (\( n_2 = 1.5 \))**: \[ 1 \cdot \sin \theta_1 = 1.5 \cdot \sin \theta_2 \] \[ \sin \theta_2 = \frac{1}{1.5}\sin \theta_1 \] - If light goes **from glass (\( n_1 = 1.5 \)) into air (\( n_2 = 1 \))**: \[ 1.5 \cdot \sin \theta_1 = 1 \cdot \sin \theta_2 \] \[ \sin \theta_2 = 1.5 \sin \theta_1 \] So, **the correct ratio is always \( n_1/n_2 \) multiplying \( \sin \theta_1 \)**. Suppose you're given an incident angle \( \theta_1 = 48.19^\circ \) **in glass** (\( n_1 = 1.5 \)), going into air (\( n_2 = 1 \)): \[ \sin \theta_2 = \frac{n_1}{n_2} \sin \theta_1 = 1.5 \sin 48.19^\circ \] \[ \sin \theta_2 = 1.5 \times 0.745 \approx 1.1175 \] You are correct: **\(\sin \theta_2 > 1\)**, which is not physically possible. This means **total internal reflection** occurs for this angle. ### Common Error If they instead did \( 1/1.5 \) at this step, that would be incorrect for light going from glass to air. That ratio is only used when light is going from air into glass. ### Summary - **You are correct**: The correct formula is \( n_1 \sin \theta_1 = n_2 \sin \theta_2 \). - When going from *glass to air*: \( \sin \theta_2 = \frac{n_1}{n_2} \sin \theta_1 \). - If \( \sin \theta_2 > 1 \), total internal reflection occurs. - **If a solution uses \( 1/1.5 \) for glass to air, that's a mistake.** Let me know if you want to see a step-by-step solution for a specific question!