1. Why does no critical angle exist for light travelling from a fast medium into a slow medium?

critical angle exists only when light attempts to travel from a slow medium (higher refractive index) to a fast medium (lower refractive index). Here's why:* **Refraction and Snell's Law:** When light passes from one medium to another, it bends (refracts). Snell's Law describes this bending: n₁sinθ₁ = n₂sinθ₂, where n₁ and n₂ are the refractive indices of the first and second mediums, and θ₁ and θ₂ are the angles of incidence and refraction, respectively.* **From Slow to Fast:** If light travels from a slow medium to a fast medium (n₁ > n₂), the angle of refraction (θ₂) is larger than the angle of incidence (θ₁). As you increase the angle of incidence, the angle of refraction also increases. At a certain angle of incidence (the critical angle), the angle of refraction reaches 90°. Beyond this critical angle, total internal reflection occurs.* **From Fast to Slow:** If light travels from a fast medium to a slow medium (n₁ < n₂), the angle of refraction (θ₂) is smaller than the angle of incidence (θ₁). As you increase the angle of incidence, the angle of refraction also increases, but it will *never* reach 90°. No matter how steep the angle of incidence, the light will always be refracted into the slower medium. Therefore, there's no critical angle in this situation. The light will always enter the slower medium, albeit at a shallower angle than the incident angle.