Electromagnetic Waves 18.1 p

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 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 temperaturewarmer 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 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 10 8 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 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 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

Assignment p538 Section Review #6, 7, 8, 9