# Geometric Optics

Principle of Least Time
 The path taken between two points by a ray of light is the path that can be traversed in the least time. Another name for the principle of least time is Fermat's principle.

## Rays

Normal
 An imaginary line perpendicular (90 °) to the surface of an optical medium.

Law of Reflection
 the angle of incidence equals the angle of reflection $\theta_i = \theta_r$

Index of Refraction
 The index of refractive index of a medium is a measure of the reduction in the speed of light inside the medium in comparison to the speed in a vacuum. Also called refractive index.

Law of Refraction
 the index of refraction in medium 1 multiplied by the sine of the angle medium 1 equals the index of refraction in medium 2 multiplied by the sine of the angle in medium 2. $n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$ light rays traveling in a high index medium bend away from the normal when refracting into a lower index medium light rays traveling in a low index medium bend towards the normal when refracting into a higher index medium

Critical Angle
 The angle where refraction occurs at 90 °. Beyond the critical angle there is total internal reflection. The critical angle only exists for rays that are in a high index medium and encounter a boundary with a lower index medium.

## Dispersion

the variation of the index of refraction with wavelength.

As a consequence rays with different wavelengths refract at different angles when traveling through a dispersive medium.