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Electromagnetic Waves
Are transverse waves consisting of changing electric fields and changing magnetic fields
They are able to carry energy from one place to another
They differ from mechanical waves in how they are produced and how they travel Review- how are mechanical waves produced?
How are waves produced? Mechanical waves are created when a source of
energy causes a vibration to travel through a medium
Electromagnetic waves are produced by constantly changing electric or magnetic fields Electric field- a region of space that exerts electric forces
on charged particles Magnetic field- a region of space produced by magnets,
changing electric fields and by vibrating charges Electromagnetic waves are produced when an
electric charge vibrates or accelerates
Think: Have you ever been warned to keep magnets away from electronic devices like a laptop? Why is this?
How do electromagnetic waves travel?
Review- mechanical waves need a medium in order to travel
Electromagnetic waves can travel through a vacuum or a medium
Electromagnetic radiation – the transfer of energy by electromagnetic waves traveling through matter or across space
The Electromagnetic Spectrum
1800- William Hershel used a prism to separate light (by wavelengths)
He wondered if there was a difference in the temperature for each color produced He set up a series of thermometers and
discovered there was a difference Lowest temperature at the blue end, highest
temperature at the red end
The visible spectrum (light human eye can detect)
Cooler temperature warmer temperature
Herschel also discovered that the temperature increased beyond the red light which led him to propose there must be invisible radiation beyond the red end (today we know this is infrared)
Later (1801) Wilhelm Ritter discovered ultraviolet radiation by experimenting with light exposed to silver nitrate dipped paper
The full range of frequencies of electromagnetic radiation is called the electromagnetic spectrum
The above diagram shows the full electromagnetic spectrum in order of increasing frequencies
The electromagnetic spectrum includes radio waves, infrared waves, visible light, UV rays, X-rays and gamma rays
- each is characterized by a range of wavelength and frequencies
Speed of electromagnetic waves
People tried to measure the speed of light but it was too fast The speed was thought to be infinite
1926- Albert Michelson measure the speed of light He placed an 8-sided rotating mirror on top of Mt
Wilson (CA) and another mirror on Mt. San Antonio 35.4km away
He timed the light beam as it traveled from one mountain to another
Since Michelson, many other scientists have measured light all confirming that light and all electromagnetic waves travel at the same speed through a vacuum Speed of light (c) = 3.00 x 108 m/s
In a vacuum, all electromagnetic waves travel at the same speed but they are not all the same
Their wavelength and frequencies vary remember speed is a
product of wavelength and frequency
Speed = wavelength x frequency
Dual nature
Electromagnetic radiation sometimes travels as waves and sometimes like a stream of particles
1801- Thomas Young showed light travels in waves by shining light through slits and observed what happened when it reached a darkened screen There was interference which only occurs when
two or more waves overlap
The photoelectric effect described in 1887 is the emission of electrons from a metal caused by light striking it Was puzzling because blue light caused it, but not a
brighter red light
1905- Albert Einstein proposed light consists of small pockets of energy now called photons Each photon’s energy is proportional to the frequency of
the light Blue light has a higher frequency than red light
Intensity
The rate at which a wave’s energy flows through a given unit of area (brightness)
The intensity of light decreases as photons travel further away from the source