Download 12.1: What are electromagnetic waves?

12.1: What are electromagnetic waves?
I. Waves in Space

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.
II. Electric and Magnetic Fields

A magnetic field surrounds all magnets.

An electric field surrounds all charges.

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.
III. Making Electromagnetic Waves

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.
IV. Properties of EM Waves

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.

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.

Frequency and wavelength are measured like other waves.
 As the frequency increases, the wavelength decreases.
V. Waves and Particles



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.
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)