Reflection, Refraction, and Snell's Law

Reflection and refraction are foundational wave optics concepts tested on the MCAT from raw Snell's law calculations to passage-based optical system design. The three mistakes that consistently trap students are: flipping the bending direction (denser medium bends light toward the normal, not away), measuring angles from the surface instead of the normal (which swaps sine and cosine in every calculation), and thinking higher refractive index means faster light (it's the opposite — n = c/v). Reflection follows a simple rule: the angle of incidence equals the angle of reflection (θi = θr), both measured from the normal. Refraction is what happens when light crosses into a new medium and changes speed — Snell's law (n1 sinθ1 = n2 sinθ2) quantifies exactly how the angle changes.

The MCAT tests this concept at every level — from raw recall of Snell's law to calculation-based problems where you solve for an unknown angle or index, to passage-based questions where you have to interpret a novel optical setup (like an endoscope or a corrective lens design) using these principles. You'll need to be comfortable with the algebra of Snell's law, but more importantly you need the physical intuition: when light enters a denser medium, it slows down and bends toward the normal; when it enters a less dense medium, it speeds up and bends away. That direction-of-bending logic shows up constantly in passage interpretation questions.

The three mistakes that consistently trap students here are (1) flipping the bending direction — thinking denser means bending away from the normal, (2) measuring angles from the surface instead of the normal, which gives you the complement of the correct angle and breaks every calculation, and (3) confusing higher refractive index with faster light when it's actually the opposite. Any one of these errors will cascade through an entire problem. Get the geometry and the physical model right first, and the math becomes straightforward.