Sunday, May 21, 2006

1

Light

a. Nature of Light

Light resembles sound in that it passes through a media; but unlike sound, it can also travel across a vacuum. This dual behavior of light, i.e. the ability to travel through a media as well as across a vacuum, has led to separate theories of its nature: wave theory and quantum theory.

Classically, light has been considered as a “stream of particles”, a “stream of waves” or a “stream of quanta”.

Physical Optics examines light as energy particles that are emitted by light sources and absorbed by other substances (Wave or Quanta Theory of Light).

  • Wave Theory helps to understand how light interacts with itself, different media and various surfaces. Wave theory allows us to understand the naturally occurring phenomena of interference, diffraction and polarization.

  • Diffraction causes a decrease in normal visual acuity for apertures less than 2 mm (such as a very small pupil of the eye).


Geometric Optics deals with the formation of images by rays of light acted on by lenses, prisms and mirrors (Particle Theory of Light).

  • The concept of vergence is the unifying concept between wave theory and geometric optics.


Quantum Optics deals with the interaction of light and matter. It considers light as having both wave and particle (photon) characteristics. When light interacts with matter, photons are emitted or absorbed.


  • Visible light is in the very narrow portion of the electromagnetic spectrum with wavelengths roughly between 400 and 800 nanometers (380 – 760nm or 4x10-6 m to 8x10-6 m). This portion of the electromagnetic spectrum represents approximately 1% of the sun’s electromagnetic spectrum that ranges from 1x10-16 m to 1x106 m.

  • Yellow light is the standard wavelength for calibration. It holds mid position in the chromatic interval of the emmetropic eye and so is in best focus.

  • A photon of wavelength 100 nm has 12.50 eV per photon. A photon of wavelength 193 nm has 6.4 eV per photon. This shows why shorter wavelengths of light (e.g. ultraviolet) have greater poten tial for photic damage, due to their higher energy level.


b. History of Light

  • Newton, in 1665 stated that light was made of particles that moved in straight lines.

  • One hundred years later, Kris tian Huygens, a Dutch mathematician, suggested that the light was a wave form, after observing that a small amount of light was always bent onto the shadow behind an opaque object.

  • Thomas Young proved the wave nature of light with a double slit defraction experiment.

  • Einstein taught that the speed of light in a vacuum is always 186,000 miles per second regardless of the speed of the observer or the source. This was proven by the Michelson-Morely experiment. Einstein’s work on light concluded that light really does act as a particle, but a particle that has wave properties.

  • The Heisenberg principle rationalizes that when you try to measure something too precisely, the act of measurement itself, changes the thing being measured. This has led to light particles being called photons or quanta. Heisenberg suggested that quanta have wave properties.

    • When light is considered as being composed of quanta, the results of all experiments and physical phenomenon can be predicted.

    • A quanta of light’s energy (E) is described by the equation E = hυ, where υ is the frequency of the light wave and h is Planck’s constant: 6.626 x 10-34 J/sec.

    • Frequency and wavelength of light are related in the equation c = w n where c = speed of light, n = frequency and w = wavelength. Therefore the constancy of the speed of light, c, guarantees a constant relationship between frequency and wavelength.



c. Movement of Light

  • Movement of light by convention is from left to right. Positive numbers measure in the direction of light, negative measure against the direction of light. Therefore, a positive lens or waveform is converging and a negative lens or waveform is diverging.

  • All naturally occurring wavefronts are diverging as they emerge from a source.

  • As light rays approach infinity, they become parallel.

  • Optical infinity is considered 20 feet or greater.

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