SPH3UW/SPH4I
Notes
Unit 10.6 Light: Wave or Particle?
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Physics Tool box
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Newton’s particle theory provided a satisfactory explanation for four
properties of light: reticular propagation, reflection, refraction, and
dispersion. The theory was weak in its explanations of diffraction and partial
reflection – partial refraction.
Huygens’ wave theory considered every point on a wave front as a point
source of tiny secondary wavelets, spreading out in front of the wave at the
same speed as the wave itself. The surface envelope, tangent to all the
wavelets, constitutes the new wave front.
Huygens’ version of the wave theory explained many of the properties of
light, including reflection, refraction, partial reflection –partial refraction,
diffraction, and rectilinear propagation.
Relationship between any point P on the nth nodal line:
The debate between the structure of light being a wave or a particle had been debated
since the early Greeks. Einstein himself finally resolved the paradox better than any Politian
trying to obtain votes.
Newton’s Particle Theory
Building on the work of Rene Descartes, Newton imagined that light consisted of streams of
tiny particles (which he called corpuscles), shooting out of a light source like bullets.
Rectilinear Propagation
The term used to describe light travelling in straight lines is easily observed by the sharp
shadows we observe and light rays breaking through the clouds. Particles obviously follow
this pattern.
Diffraction
Newton noticed that light does not travel “around a corner”. But explained the diffraction
that has a wave-like property, as collisions between the light corpuscles at the edge.
Reflection
Newton demonstrated that under the law of perfect elastic collisions, the law of reflection
follows from the law of motion
Refraction
Newton was able to demonstrate the nature of refraction with the particle model. His
theory stated that a particle will bend toward the normal if there speed increases. For
example if a ball is rolled down a ramp from a raised horizontal surface to a lower
horizontal surface, it will bend (refract) toward the normal. But is 1850 Jean Foucault
demonstrated that the speed of light in water was less that speed of light is air – the
opposite to Newton’s particle theory. Oops!
SPH3UW/SPH4I
Unit 10.6 Light: Wave or Particle?
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Partial Reflection – Partial Refraction
When light refracts, some of the light is reflected. Newton had a problem explaining this
property in his particle theory. He attempted by a “theory of fits” . Sometimes a particle
hits the surface a fits into the reflection angle or fits into the refraction angle. Very weak!
Dispersion
When white light passes through a prism, different wavelengths are refracted through
different angles, creating a spectrum of colours. This phenomenon is called dispersion..
Newton proposed that each particle in the spectrum comes from a light particle that has
different mass. The lower masses, having less momentum, would be diverted (refracted)
more easily.
Even though Newton was a person of esteemed reputation, he still considered his particle
(as the as the wave theory) as hypotheses. A theory that required more testing.
Huygens’ Wave Model
Initially developed by Robert Hooke in 1665, Huygens developed the theory 20 years later,
creating a theory of light as a wave. (We still uses the Huygens’ principle as a diagram aid
from predicting the position of the wave front.
SPH3UW/SPH4I
Unit 10.6 Light: Wave or Particle?
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Huygens’ Principle
Huygens’ wave theory considered every point on a wave front as a point source of tiny
secondary wavelets, spreading out in front of the wave at the same speed as the wave
itself. The surface envelope, tangent to all the wavelets, constitutes the new wave front.
Rectilinear Propagation
Wave theory treats light as a series of wave fronts perpendicular to the path of the light
rays. Huygens thought of the rays as simply representing the direction of motion of a wave
front.
Diffraction
If light was a wave, then the waves must bend around a corner. For example, if two rooms
are connected by an open doorway and a sound is produced in a remote corner of one of
them, a person in the other room will hear the sound as if it originated at the doorway. As
far as the second room is concerned, the vibrating air in the doorway is the source of the
sound. The same is true of light passing the edge of an obstacle, but this is not as easily
observed because of the short wavelength of visible light.
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Unit 10.6 Light: Wave or Particle?
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Reflection
Waves obey the laws of reflection from optics. In each case the angle of incidence equals
the angle of reflection for both straight and curved reflectors
Refraction
Huygens, using his wavelet model, predicted that light would bend toward the normal as it
passes into an optically denser medium
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Unit 10.6 Light: Wave or Particle?
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Partial Reflection – Partial Refraction
The ripple tank demonstrates that waves reflect and partially refract whenever there is a
change of speed, and that the amount of partial reflection varies with the angle of
incidence.
SPH3UW/SPH4I
Unit 10.6 Light: Wave or Particle?
Extra Notes and Comments
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