Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 4: Light - Wave or Particle?

Reference: Chapter 9.4

Scientific Model:

~ A good scientific model should be based on a few easily understood assumptions

~ It should be able to predict outcomes and properties that can then be experimentally verified.

Light  - Wave or Particle?

~ Newton was the principal advocate of the Particle Theory

~ Christian Huygens and Robert Hooke were advocates of the Wave Theory

~ The debate lasted over 100 years

Newton’s Particle (Corpuscular) Model of Light:

Newton originally believed that light behaved as particles.  Newton assumed that:

1. All sources emit “light particles”

2. The particles are extremely small
3. The particles travel at very high speeds

 In most cases, his assumptions seemed to work in explaining how light behaved. The first three properties of light below, the particle model could explain well, however points 4 to 6 (Partial Reflection/Partial Refraction, Refraction, Diffraction) the particle model could not explain very well. The explanations of light based on his particle theory were as follows:

1. Explanation of the Rectilinear Propagation (means moving in a straight line) Property.

            The rectilinear propagation properties of light could be explained by the particle theory. The particle theory said that light was small,

            fast particles so there would be a negligible effect of gravity & at fast speeds the light would go in a straight line.

2. Explanation of Reflection.  The particle theory explained this well (i.e throw a ball at the blackboard at an angle and notice that it does indeed bounce off at an angle.  The ball represents a particle.)

3. Explanation of Dispersion. (dispersion - when white light passes through a glass prism, different wavelengths are refracted through different angles, generating a display of colours). Newton's particle model was sufficient in explaining this by saying that each particle in the spectrum had a different mass.  He suggested that the lower mass particles would get diverted more easily (i.e. red particles he suggested would have a higher mass than blue particles). 

4. Partial Reflection-Partial Refraction

Newton's particle theory could not explain this very well.  He proposed that some particles hit the surface and get reflected and some particles hit the surface and get refracted.

5. Explanation of Diffraction. The particle theory did not provide a good explanation of diffraction.  Newton argued that light does not travel around a corner as waves do. Newton thought that the effect around a barrier was just particles colliding at the edges rather than from the outward spreading of waves.

6. Explanation of Refraction.  This is where the particle theory did not work in explaining light.  Newton believed that water attracts approaching particles of light in much the same way that gravity attracts a rolling ball on an incline (Remember the ramp demo we did in class where a ball rolled at a transverse angle down a ramp from a raised surface to a lower surface bent toward the normal and increased its speed). Because of this he believed the following  particle model’s explanation of refraction:

          As particles of light travel from 1 medium to another at oblique incidence, they encounter a net force at the boundary (similar to the

          force experienced by a ball going down a ramp) that,

• Speeds up the particles of light

• Pushes them to take a direction closer to the normal

Based on THIS explanation it was suggested that the speed of light in refractive media such as glass & water is greater than the speed of light in air (which we now know is not true).  At the time no one could measure the speed of light in different media, so everyone believed Newton’s particle theory.  However, later experiments showed the opposite and so Huygen’s wave model started to be taken more seriously.

 

Huygens' Principle:

Every point on a wave front can be considered as a point source of tiny secondary wavelets that spread out in front of the wave at the same speed as the wave. The surface envelope, makes the wave front.

His model predicted that light would bend toward the normal as it passes into a denser medium because its speed is slower.

 

Suggested Work:

Be able to answer the following:

1. Describe how the Particle Model accounted for: a) the Rectilinear propagation of light, b) Reflection, c) Refraction

2. Does the Particle Model merit being categorized as a good model?

3. Why was the Particle Model finally abandoned.  Give a detailed anser.