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Speed of light

Speed of light. Galileo – “Its fast” Roemer (~1700) – Period of Jupiter’s moon “Its fast, but finite” Michelson (1880-1920) – First experimental measurement of the speed of light. Speed of light in materials. For free space (vacuum) we showed. We expect in any other material.

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Speed of light

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  1. Speed of light • Galileo – “Its fast” • Roemer (~1700) – Period of Jupiter’s moon • “Its fast, but finite” • Michelson (1880-1920) – First experimental measurement of the speed of light.

  2. Speed of light in materials For free space (vacuum) we showed We expect in any other material

  3. Speed of light in materials (dielectrics) For a dielectric material we showed We observe

  4. Index of refraction We will see later that n varies with frequency of light

  5. Ray model of light Incident Ray Reflected Ray ni nt Transmitted Ray Rays are a construct to show the direction the wave is traveling Rays are generally perpendicular to the surfaces of equal phase (wave fronts).

  6. The Laws that are Empirically Observed • Law of reflection. • Snell’s Law of Refraction.

  7. Reflection Definitions e.g. air to glass • External • Internal • Specular - Reflection off a smooth surface • Diffuse - Reflection off a rough surface e.g. glass to air

  8. The image is the same size as the object Plane Mirrors Virtual Image

  9. How big does your bathroom mirror have to be? h Shift your observation point to a top view and observe the right-left inversion of an image in a plane mirror.

  10. Corner Reflectors • The reflected light ray is always parallel to the incident ray. • Implication for stealth technology.

  11. A More Physical View of the Law of Reflection C B ni A nt D

  12. A More Physical View of the Law of Reflection C B ni A D nt

  13. Paraxial rays

  14. Refraction • The bending of a light ray at the boundary between two media. Snell’s Law:

  15. Refraction Basics • Incident, reflected, and refracted rays are all in the plane of incidence. • A ray entering a higher index medium bends toward the normal. • A ray entering a lower index medium bends away from the normal. • Relative index of refraction • Vacuum wavelength

  16. Example • A light ray is incident on a water surface at an angle of 30 degrees w.r.t. the normal to the surface. What is the angle of the refracted ray relative to the normal to the surface? • Does the frequency of the light change at the water boundary? • Does the wave length change in the water? • How much?

  17. Fermat’s Principle • The actual path between two points taken by a beam of light is the one that is traversed in the least time. S h ni O x nt b P a-x a

  18. Total Internal Reflection There will always be an angle such that (even though it can’t be physically) We define a critical angle for the limiting case:

  19. Fiberoptics nc ni nf How big can be?

  20. Fiberoptics nc ni nf How big can be?

  21. Dispersion (or wavelength) • Difference in wave speed (n) in a material due to frequency

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