1 12.1: What are electromagnetic waves?

 Electromagnetic waves:  made by vibrating electric charges  can travel through space (don’t need matter)  Travel by transferring energy between vibrating electric and magnetic fields. 2

 A magnetic field surrounds all magnets.  An electric field surrounds all charges. 3

 Magnetic and electric fields:  Exert forces without having to touch objects  Enable magnets and charges to exert forces at a distance.  Exist even when there is no matter (in space)  Electric charges can also be surrounded by magnetic fields.  All moving electric charges are surrounded by a magnetic field.  The motion of electrons creates a magnetic field around the object they flow through.  A change in one field causes a change in the other. 4

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 Recall: EM waves are made by a vibrating charge.  This means that a vibrating charge has both an electric field and a magnetic field.  As the charge vibrates, the electric and magnetic fields change.  A vibrating electric charge creates an EM wave that travels outward in all directions from the charge.  EM waves are transverse waves because the electric and magnetic fields vibrate at right angles to the direction the wave travels. 6

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 All objects emit EM waves!  The wavelengths of the EM waves emitted shorten as the temperature of the material increases.  The energy carried by an EM wave is called radiant energy.  EX: sunlight causes the electrons in your skin to vibrate and gain energy. 8

 All EM waves travel at 300,000,000 m/s in a vacuum (space), i.e. “the speed of light.”  Nothing travels faster!  EM wave speed through matter depends on the material. 9

 Frequency and wavelength are measured like other waves.  As the frequency increases, the wavelength decreases. 10

 We define a wave as a disturbance that carries energy and a particle as a piece of matter.  EM waves can behave as both a wave and a particle (the particle-wave duality).  Phenomenon discovered in 1887 by Heinrich Hertz.  Experiment known as the “photo-electric effect”:  Shine a light on metal and the metal will eject electrons.  Whether it happened depended on the frequency of the light, not the amplitude.  Einstein later explained this: electromagnetic waves can behave as a particle, called a photon, whose energy depends on the frequency of the waves. 11

 It is now known that all particles can behave like waves.  The figures below demonstrate particles experiencing diffraction when encountering an obstacle (electrons on the left, water on the right) 12