Reflection: Angle of incidence = angle of reflection, both measured from the normal.
Snell's law: n₁ sin i = n₂ sin r. From air into a material, n = sin i ÷ sin r.
Critical angle: sin C = n₂/n₁ when n₁ is greater than n₂. Beyond C, total internal reflection occurs.
Optical fibre: Light can remain trapped in a higher-index core through repeated total internal reflection when the incidence condition is satisfied.
Converging lens: A ray parallel to the principal axis passes through the far focal point. A ray passing through the optical centre continues undeviated in the thin-lens approximation. Their intersection locates the image.
Assumptions: Plane optical boundaries, uniform isotropic materials, ideal thin lenses, paraxial lens rays and monochromatic light. Distances are in cm; angles are in degrees (°).