PHY 113 Lecture Notes - Lecture 20: Heinrich Hertz, James Clerk Maxwell, Electromagnetic Spectrum

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11 Jun 2018

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Electromagnetic Spectrum
Starting with Thomas Young’s demonstration of the diffraction and interference of light in
1801, the evidence gathered during the 1800s supported the wave theory of light. Diffraction
and constructive and destructive interference are fundamental properties of waves. Particles
do not diffract or demonstrate constructive and destructive interference. So Young’s
demonstration was conclusive evidence that light energy travelled as waves. The
demonstration of polarization of light was not only evidence that light travelled as waves, but
as transverse waves-remember that only transverse waves can be polarized.
However, even with this conclusive evidence of diffraction, interference and polarization,
scientists were reluctant to give up on the particle model of light. The reason for this
reluctance was that the wave model of light could not explain how light travels through
In the 1800s, all waves were thought to be mechanical waves. In the mid 1800s James Clerk
Maxwell came up with the theory of electromagnetic waves. His theory was that not all
waves are mechanical, but another kind of wave existed-electromagnetic waves.
Electromagnetic waves do not require a physical medium and therefore can travel through
space. They can travel through space by creating magnetic and electric fields in space.
Today, it is known that there is a broad range of frequencies of electromagnetic radiation
which make up the electromagnetic spectrum. All waves in the electromagnetic spectrum
have the same basic characteristics as visible light.
It can be seen from the above chart that the light humans see is a very small part of the
electromagnetic spectrum.
Now, this is not the end of the story of the nature of light. Today it is believed that light
travels as an electromagnetic wave but also that light has a particle nature as well. In the late
1800s, German physicist Heinrich Rudolf Hertz discovered the photoelectric effect, where
certain metals would emit electrons when struck by light. The photoelectric effect can only be
explained by assuming that light exists as particles-massless lumps of energy called photons.
Light is considered to have a dual nature.
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