1. Let's explain the concepts of reflection, refraction, and total internal reflection (TIR). 2. Reflection occurs when light bounces off a surface, e.g., light hitting air-aluminium or air-glass boundary reflects some portion of light. 3. Refraction is the bending of light as it passes from one medium to another with a different refractive index, e.g., from air (n=1.00) to glass (n ≈ 1.5). 4. Total internal reflection occurs when light tries to move from a denser medium to a less dense medium (like glass to air) at an angle greater than the critical angle, causing all light to reflect inside the denser medium. 5. To calculate the critical angle $\theta_c$ for total internal reflection between glass and air, use Snell's Law: $$n_1 \sin \theta_c = n_2 \sin 90^\circ$$ 6. Since $\sin 90^\circ = 1$, $$\sin \theta_c = \frac{n_2}{n_1}$$ where $n_1$ is refractive index of glass (~1.5) and $n_2$ is refractive index of air (1.0). 7. Therefore, $$\theta_c = \sin^{-1} \left( \frac{1.0}{1.5} \right) = \sin^{-1} (0.6667) \approx 41.8^\circ$$. 8. For aluminium, which is a metal and typically opaque with complex refractive indices, total internal reflection in the classical sense does not occur; light mostly reflects at the surface. 9. Summary: - At air-aluminium, mostly reflection. - At air-glass, partial reflection and refraction. - At glass-air, total internal reflection if angle of incidence is greater than $41.8^\circ$. Final answers: - Critical angle for glass to air is approximately $41.8^\circ$. - Total internal reflection happens only when light travels from glass to air beyond that angle.