Postulates of Ray Optics

The following principles of ray optics may be used to describe many optical systems. The numbering system is of no significance.

1. Light travels in the form of a ray. This means that light will travel from a source and is observed when reaching a detector.

2. Optical rays are vector which point in the direction of energy flow.

3. An optical medium is characterized by a refractive index, n = c0 / c, where c0 is the speed of light in free space and c is the speed of light in the medium. The time taken by light to travel a distance d is d/c = nd/c0. The optical pathlength is n*d.

4. In an inhomogeneous medium, the refractive index n(r) is a function of the position r(x,y,z). The optical pathlength along a path between A and B is the integral of A to B of n(r)*ds.

5. Fermat’s Principle states that optical rays travel from A to B following the path that requires the least amount of travel time.

6. Hero’s Principle states that light travels in straight lines in a homogeneous medium. A homogeneous medium means that the refractive index is consistent throughout.

7. Light reflects from mirrors in accordance with the law of reflection: The angle of reflection equals the angle of incidence and the reflected ray lies in the plane of incidence. This may be proven using Hero’s principle.


8. At a boundary between two mediums of different refracting indexes, a ray is split in two. One resulted ray is a reflected ray and the other is a refracted or transmitted ray. The reflected ray is shown in figure (b) above as vector C, while the refracted ray is C’.

9. The refracted ray lies in the place of incidence. The angle of refraction is related to the angle of incidence by Snell’s Law:


10. The proportion of reflected light to refracted light is not dealt with in ray optics.


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