Physics Quiz description: Think of optical density as the drag coefficient for light. It’s basically just how much a material kills light’s speed. You see a higher refractive index? You’re looking at higher optical density. Keep in mind, mass density—mass per unit volume—is a different animal entirely. Don’t mix them up. Total internal reflection. That’s the trick. It hits when light tries to bail from a dense medium, like glass, into something thin, like air. If the angle of incidence gets past that specific “critical angle” point, light quits refracting. It just bounces back. Total lockdown. The critical angle is just the cutoff—the exact minimum angle where the light skims the border at 90 degrees. Take diamond. It’s got an index around 2.42. Because it’s so dense, the critical angle is tiny—only about 24.4°. Reflection is pretty rigid. The second law says the incident ray, the reflected ray, and that normal line all hang out in the same plane. If they didn’t? Mirrors would be useless. Everything would be a distorted, glitchy mess. Combine that with the first law—angle in equals angle out—and you’ve got a system that actually makes sense. Works for curved surfaces too, not just flat ones. A few quick definitions: Diffraction is just sound bending around a corner or a hole. Refraction? That’s sound changing direction when it shifts from, say, air into water. Transmission is just the sound wave getting from A to B. Simple enough. Finally, throw a charged particle into a magnetic field at a perpendicular angle and it’s going to get pushed around. The force—always perpendicular to the velocity and the field, thanks to the Right-Hand Rule—forces it into a curve. If nothing changes? It’s just going to spin in circles forever.