Theories Of Light
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In the seventeenth century two rival theories of the nature of light were proposed, the wave theory and the corpuscular theory.
The Dutch astronomer Huygens (1629-1695) proposed a wave theory of light. He believed that light was a longitudinal wave, and that this wave was propagated through a material called the 'aether'. Since light can pass through a vacuum and travels very fast Huygens had to propose some rather strange properties for the aether: for example; it must fill all space and be weightless and invisible. For this reason scientists were sceptical of his theory.
In 1690 Newton proposed the corpuscular theory of light. He believed that light was shot out from a source in small particles, and this view was accepted for over a hundred years.
The quantum theory put forward by Max Planck in 1900 combined the wave theory and the particle theory, and showed that light can sometimes behave like a particle and sometimes like a wave. You can find a much fuller consideration of this in the section on the quantum theory.
Wave theory of Huygens
As we have seen, Huygens considered that light was propagated in longitudinal waves through a material called the aether. We will now look at his ideas more closely.
Huygens published his theory in 1690, having compared the behaviour of light not with that of water waves but with that of sound. Sound cannot travel through a vacuum but light does, and so Huygens proposed that the aether must fill all space, be transparent and of zero inertia. Clearly a very strange material!
Even at the beginning of the twentieth century, however, scientists were convinced of the existence of the aether. One book states 'whatever we consider the aether to be there can be no doubt of its existence'.
We now consider how Huygens thought the waves moved from place to place.
Consider a wavefront initially at position W, and assume that every point on that wavefront acts as a source of secondary wavelets. (Figure 1
In the seventeenth century two rival theories of the nature of light were proposed, the wave theory and the corpuscular theory.
The Dutch astronomer Huygens (1629-1695) proposed a wave theory of light. He believed that light was a longitudinal wave, and that this wave was propagated through a material called the 'aether'. Since light can pass through a vacuum and travels very fast Huygens had to propose some rather strange properties for the aether: for example; it must fill all space and be weightless and invisible. For this reason scientists were sceptical of his theory.
In 1690 Newton proposed the corpuscular theory of light. He believed that light was shot out from a source in small particles, and this view was accepted for over a hundred years.
The quantum theory put forward by Max Planck in 1900 combined the wave theory and the particle theory, and showed that light can sometimes behave like a particle and sometimes like a wave. You can find a much fuller consideration of this in the section on the quantum theory.
Wave theory of Huygens
As we have seen, Huygens considered that light was propagated in longitudinal waves through a material called the aether. We will now look at his ideas more closely.
Huygens published his theory in 1690, having compared the behaviour of light not with that of water waves but with that of sound. Sound cannot travel through a vacuum but light does, and so Huygens proposed that the aether must fill all space, be transparent and of zero inertia. Clearly a very strange material!
Even at the beginning of the twentieth century, however, scientists were convinced of the existence of the aether. One book states 'whatever we consider the aether to be there can be no doubt of its existence'.
We now consider how Huygens thought the waves moved from place to place.
Consider a wavefront initially at position W, and assume that every point on that wavefront acts as a source of secondary wavelets. (Figure 1