Electromagnetic Waves

18.1

p 532-538

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

Assignment

p538 Section Review #6, 7, 8, 9