PHY 113 Lecture Notes - Lecture 18: Electromagnetic Radiation, Longitudinal Wave, Radiography

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11 Jun 2018
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Waves
Energy travels by either waves or particles. If you get hit with a ball at a ball game, the
energy was transferred from the bat to you by means of a particle. When you hear the crack
of the bat against the ball that energy travelled from the bat to you as a wave.
A wave is a disturbance that carries energy from one point to another without the
transmission of matter.
Types of Waves
Mechanical waves require a medium to travel.
Electromagnetic waves can travel in a vacuum.
Mechanical Waves
Mechanical waves need something to travel through. They propagate through the vibration of
particles of the material through which they are traveling. There are standing waves, where
the wave remains in a fixed position, and traveling waves, where the wave travels away from
its source.
Electromagnetic Waves
Until Maxwell’s predictions regarding electromagnetism, all waves were assumed to be
mechanical. Electromagnetic waves, by their nature, move on their own, without the need for
a medium to carry them. Radio waves, microwaves, light, and X-rays, are all types of
electromagnetic waves.
Waves are also classified as longitudinal and transverse. In mechanical waves the medium is
vibrating. This vibration can be parallel to the direction of energy flow, or it can be
perpendicular to the direct of energy flow.
If the vibration of the medium is parallel to the direction of energy flow, the wave is called a
longitudinal wave. In a longitudinal wave, the wave is composed of compressions and
rarefactions. The compressions are regions in the wave where the medium is compressed.
The regions in the medium which are stretched out are called rarefactions.
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