Light can be thought of as a stream of particles because it shows similar behaviors in some experiments.
Isaac Newton (~1665) made up a particle model of light to explain many of the known behaviors of light at that time. He was able to explain:
- Straight line propagation of light (Free particles move in a straight line)
- The intensity of light (Number of particles in the stream)
- Different colours (Different types of particles)
- Reflection of light from flat and curved surfaces (Particles hitting a solid surface follow the law of
reflection, Ði = Ðr )
- Refraction (change in direction) of light as it crosses the interface between two media.
The setup demonstrating the refraction of light consists of two horizontal levels connected with a sloping surface. A ball bearing rolls along the top level and down the slope to the lower level with a greater speed. A change in direction (towards the normal) is clearly observable. However, this demonstration fails to show that the speed of light decreases when it enters a denser medium (represented by the lower level).
He was unable to explain:
- Partial reflection and partial transmission of light at an interface
- The existence of Newton’s rings and other related phenomena due to the interference of light.
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