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(3) Optics and Light: A problem of key importance at the time of Newton was the nature of light - his conclusions were very different from those of Huyghens.
(i) In the Newtonian picture light, like matter, was made from tiny corpuscles having mass. Explain in general terms how Newton thought about the motion of light.
What caused refraction and reflection, what happened to the light corpuscles during this process, and why was it that some light corpuscles were refracted and others reflected? What was it that kept light corpuscles moving, and how fast did they move; and what did they interact with? The key role of the aether, and how Newton thought about it, should be explained here. Finally, explain what you think were the weaknesses of this theory (without assuming things that were not known at the time of Newton).
In 1704, Newton published his Opticks. He proposes that light consists of billions of little corpuscles or masses that cannot be seen. Today we know that in one microsecond, light travels 300 meters!
As many other thinkers of his day, Newton believed that light is made of different particles 3⁄4 light corpuscles 3⁄4 that are very small bodies, moving extremely fast in straight lines, or rectilinearly, through a uniform medium, while interacting with it. He believed that a constant force acted on the light rays accelerating the light particles extremely rapidly to a really high velocity, so fast, in fact, the light corpuscles could only travel in straight lines at a finite, but very high velocity.
Newton argued that if the light corpuscles journeyed from one medium to another, this constant force acted normal to the interface, and then this force changed the light corpuscles’ direction in a perpendicular way. In other words, a force acts on light particles at the interface. According to Newton’s First Law, if no force acts on the light corpuscles, then the light particles would go on travelling along in a straight line given that no force acts on them. Moreover, as the light corpuscles interact with the uniform medium and with the force that accelerates them, they reach a terminal velocity given that there is a resistance by the medium following Newton’s 3rd Law, that for every force acting on a body, there is an equal and opposite reaction, acting somewhere.
Because Newton felt this medium could not be another particle, for if it were, there would be an infinite regress, he invoked the aether as the uniform medium that permeated all things. As such, the aether was everywhere, including in the vacuum, but its density was not uniform. In the vacuum, aether was at its highest density; in solid bodies and in the pores or tiny spaces between solid bodies, the aether was at its lowest density. The aether, a continuous medium able of supporting wave-like disturbances propagating through it, was some kind of invisible fluid in which pressure differences drove motion. Like Huyghens, he realized that for the aether to cause motion, it needed to be a very stiff medium to avoid compression under large pressures. Through the stiff aether, these waved
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