incident angle

Meaning: The incident angle (θ₁) is the angle between the incoming ray and the normal at the point of contact with a boundary. A ray travelling along the normal has θ₁ = 0° and passes straight through without bending. In reflection, the reflected ray leaves at an angle equal to θ₁. In refraction, a ray entering a medium with a higher index of refraction bends toward the normal, and one entering a medium with a lower index bends away from it.

Equation: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the indices of refraction of the first and second media (unitless), θ₁ is the incident angle and θ₂ is the refracted angle (both in radians in SI, often given in degrees). Law of reflection: θr = θ₁. Critical angle: sin θc = n₂/n₁, valid when n₁ > n₂.

On the MCAT: Passages show a ray crossing from one medium to another, such as air to water or glass to air. You predict whether the ray bends toward or away from the normal, compute θ₂, or decide whether total internal reflection occurs. Total internal reflection requires going from higher n to lower n with θ₁ greater than θc. For curved mirrors, the normal lies along the radius through the point of contact.

Common confusion: Students measure the angle from the surface. The incident angle is always measured from the normal, so if a problem gives the angle to the surface, subtract it from 90°.