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# 4: Geometric Optics

• 4.1: Overview
Rays, or beams of light, can travel in three ways: directly from a source, through a material, or indirectly by reflection.
• 4.2: Reflection, Refraction, and Dispersion
The law of reflection states that the angle of reflection equals the angle of incidence.
• 4.3: Lenses
Ray tracing is the technique of determining the paths light rays take; often thin lenses (the light ray bending only once) are assumed.
• 4.4: Mirrors
A mirror is a reflective surface that bounces off light, thus producing a real or virtual image.