Waves and Light

• Light – a form of energy
• Two ways that energy can travel
  • As particle motion
  • As wave motion
• Early view of light differed:
  • Plato thought light was streamers
  • Pythagoreans thought light was a particle
  • Empedocles thought light was a wave
  • Newton (1700’s) thought light was a particle
  • Huygens (1700’s) thought light was a wave
• By 19^th century, there was overwhelming evidence to support the wave model of light. To understand why, one must realize that all theories should do two things:
  • Explain all known observations
  • Predict new behaviours that have not been observed.

The Corpuscular Theory – Newton’s Theory

• Newton thought that light was made of particles (corpuscules) that emanated from the light source.
• Rectilinear propagation – light travels in straight lines.
  • Evidence for this is the presence of sharp shadows.
  • A ball thrown does not have rectilinear propagation, because gravity affects the direction of motion. Newton had to explain why light did not behave in the same way. He felt that the speed of light is extremely high, and so deviations from rectilinear motion were not noticeable. He also thought the mass of the particles was very small.
• Diffraction
  • Newton felt that light does not travel around corners. Any observed effects of this, Newton explained by the interaction of particles when they run into each other at the edges of objects.
• Reflection
  • Newton demonstrated that in perfectly elastic collisions, the laws of reflection could be derived from the laws of motion.
  • A particle incident on a surface has components of its velocity in both the X and Y directions. If the force on the particles acts perpendicular to the surface, then the X component remains unchanged. Because kinetic energy is conserved, the Y component of velocity is just reversed. This results in the angle of incidence being equal to the angle of reflection.
• Partial reflection and refraction
  • When light refracts, some light will be reflected. Newton explains this by his “Theory of Fits.” Some particles, when encountering an object, would “fit” between the atoms and some would not. Those that didn’t “fit” would reflect.

• Dispersion – the separation of light into its colours.
  • Newton explained this by saying that different colours had particles of different mass. Particles of different mass would be affected differently when refracted. Less massive particles would be affected the most, and as a result, would refract the most
• Refraction
  • It can be demonstrated that light bends towards the normal when moving from air to water. Newton believed that water attracted light particles, therefore, the particle’s speed increased. This increase in speed would cause the particle’s path to bend towards the normal.
  • This view predicts that the speed of light in water was greater than in air. In 1850 Foucault showed that LIGHT TRAVELLED SLOWER IN WATER THAN IN AIR. The particle theory must, therefore, be WRONG.
• Summary
  • Newton’s theory gave strong explanations for reflection, refraction, dispersion and straight-line transmission, but was weak on diffraction as well as for partial reflection and refraction.
  • Evidence was eventually collected which his theory could not explain, and therefore, had to be abandoned.

The wave Theory of Light

• This theory was proposed by Hooke (1665), and improved by Huygens.
• Huygens’ principle – each point on a wave, behaves as a point source for waves in the direction of wave motion. The line tangent to these circular waves is the new position of the wave front a short time later.

• The superposition principle explains why we don’t see the circular waves.
• Huygens’ wave model was able to explain reflection, refraction, partial reflection and refraction, and diffraction. Huygens’ theory had difficulty with rectilinear propagation (the reason Newton rejected waves).
• Reflection
  • The information in chapters 8 and 11 explains how light obeys the laws of reflection.
• Refraction
  • Huygens used his principle to predict that light is bent towards the normal as it passes into an optically denser medium (speed slows down). This was explained to you in chapter 8.
  • Foucault’s test, in which he measured the speed of light in water, showed that HUYGENS WAS CORRECT and Newton was wrong.
  • Snell’s law (the law of refraction) holds for light.
• Partial reflection – partial refraction
  • Mechanical waves show partial reflection and refraction whenever there is a change in velocity. The strength of the effect is dependant upon the angle of incidence, and is strongest for waves moving from a slow to a fast medium.
  • Light shows this partial reflection and refraction. This supports the idea that light is a wave.
• Diffraction
  • As previously discussed, the amount of diffraction is dependant upon the relationship between the opening size and wavelength. Once the wavelength of visible light was determined, it became obvious that the wave theory was better than the particle theory for explaining diffraction.
  • Wavelengths are 4 X 10^-7 m for blue light to 7 X 10^-7 m for red light.
• Rectilinear propagation
  • Newton’s corpuscular theory did a better job of explaining rectilinear propagation than did Huygens’ wave model. Once the wavelength of visible light was determined, it became evident that for such small wavelengths, very little diffraction would occur. This meant that rectilinear propagation was no longer a problem for the wave model of light.

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November 21, 2013