Properties of Light

• What is light? A form of energy.
• There has been many views of what light is
  • Pythagorus thought light was a stream or particles
  • Aristotle thought light was a wave
  • Newton thought light was a particle.
• Generally, two views evolved: light is
  • A particle or
  • A wave

The Transmission of light

• Rectilinear propagation – The movement of something in straight lines.
  • Light has this kind of propagation
  • Evidence for this is the existence of sharp shadow edges.
  • There are exceptions to rectilinear propagation of light.
    • Diffraction, refraction, reflection etc.
• Beam – a stream of light rays
  • Represented diagrammatically by a number of rays (often arrows).
• Rays do not adequately explain light’s nature, but it is useful for explaining reflection, refraction and image formation etc.
• Geometric optics – explanations involving the geometric analysis of rays.

The Speed of Light and the Index of Refraction

• Light travels at very high speed.
• Galileo made the first attempt to measure the speed of light by using hooded lanterns on two different hilltops.
  • Two people practiced ahead of time. One would lift the hood off his lantern, and the other would react as quickly as possible to this by pulling the hood off of his lantern.
  • Then they went out and stood on two hills that were separated by some known distance. One person was on one hill and the other on the other hill. One person lifted the hood off of his lantern and when the second person saw the light from this lantern, he lifted the hood off of his lantern. Galileo used a water clock to try to measure the time for the light to travel to the second hill and back again.
• Reaction time prevented good results for Galileo.
• Roemer was the first to show that light did not travel instantaneously.
  • He used one of Jupiter’s moons which has a period of 42.5 h, to predict when future eclipses would occur.
  • He noted that eclipses occurred progressively earlier than predicted as the Earth got closer to Jupiter (and vise versa).
  • He reasoned that less distance or more distance accounted for the late or early eclipses respectively.
  • Roemer determined that light took 22 minutes to cross Earth’s orbit. (1320 s)
  • Huygens calculated the diameter of Earth’s orbit and used Roemer’s data to calculate the speed of light. He estimated Earth’s orbital diameter to be 3.0 X 10¹¹ m.

• Huygen’s and Roemer’s work was at first rejected.
• In 1905 Michelson used a rotating mirror to calculate the speed of light.

• In the above diagram, while the light traveled from the rotating mirror to the stationary mirror and back, the rotating mirror rotated so that the light travels from A to the stationary mirror to H. The following calculations are made in order to calculate the speed of light.

• In 1933 Michelson tried to re-measure C (the speed of light) with an evacuated tube 16 km long. He died before the experiment was completed.
• The accepted value of C = 2.997 924 562 X 10⁸ m/s + or – 0.000 000 011 X 10⁸ m/s.
  • For our purposes, C = 3.00 X 10⁸ m/s is accurate enough.
• The speed of light in air is very nearly the same as in vacuum; therefore, we use the same value for both.
• Light travels at significantly different speeds in other media (materials like glass).
  • Foucault measured the value of C in water to be about 0.75 C in air.
• All substances have C < C in vacuum.
• Optically dense – media that have a speed of light which is less than in vacuum.
  • The denser, the less C is.
• Absolute Index of Refraction – the ratio of the speed of light in vacuum to the speed in a given material.

• n = the absolute index of refraction, c = the speed of light in vacuum, v = the speed of light in the material.
• The higher n is the slower the light travels in the substance.
• For light moving from air into a material we have

and therefore, in most cases, differences can be ignored.

• Relative Index of Refraction – the ratio of the speed of light in medium 1 to the speed in medium 2.

• n₁₂ = relative index of refraction, v₁ = speed of the wave in medium one, v₂ = speed of the wave in medium two, n₂ = absolute index of refraction for medium two, and n₁ = absolute index of refraction for medium one.

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November 18, 2013