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. http://upload.wikimedia.org/wikipedia/en/e/eb/Rays_incident_on_medium.png

Law of Reflection

the angle of incidence equals the angle of reflection [math]\theta_i = \theta_r[/math]

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. [math]n_1 \sin(\theta_1) = n_2 \sin(\theta_2)[/math] 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.

Click the show/hide link to view the dispersion curves for common glasses
http://upload.wikimedia.org/wikipedia/en/2/20/Dispersion-curve.png

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

Click the show/hide link to view the dispersion visualisation
http://upload.wikimedia.org/wikipedia/commons/8/8b/Light_dispersion_conceptual.gif