# Electromagnetic Waves and Light

## Presentation on theme: "Electromagnetic Waves and Light"— Presentation transcript:

Electromagnetic Waves and Light
EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light

. What is Light? The ancient Greeks thought that light was a particle
Later scientists believed that light traveled in waves Einstein showed that light does have particle characteristics So which is it: wave or particle? The scientific community now agrees that light can be thought of as both a wave and a particle, because it shows behaviors of both. .

Light as a Particle Seems to travel in a straight line
Light seems to bounce off objects like a small particle would do Experiments show that it’s quantized (that is, at very, very low levels, it can be described by integer units, like we can do with parts of an atom) The elementary particle is called a photon

How is it a wave? A water wave, sent through a hole, will curve around the edges-- diffraction Same deal for light Lots of other ways Whether we think of it as a wave or a particle depends on the situation

Light is an Electromagnetic Wave
Visible light is just one example of Electromagnetic Waves Visible Light They all move at the same speed in a vacuum …that produces transverse electric and magnetic field waves They start with an accelerating charge.. Electric Field Accelerating Charge Magnetic Field

What are electromagnetic waves?
The changing electric field produces a changing magnetic field perpendicular to it Electric Field Accelerating Charge The two waves travel together as one “Electromagnetic Wave” Magnetic Field Since it’s a field wave, NOT mechanical, it can travel in a vacuum

History of the Speed of Light
Ancient Greeks believed the speed of light to be infinitely fast. 1675: Olaus Roemer estimated the speed of light using observations of Jupiter’s moons In 1880, Albert Michelson crafted an ingenious experiment using rotating mirrors to measure the speed of light more precisely.

Difference indicates time for light to get to Earth
Roemer’s Observation How long is a moon of Jupiter hidden? Jupiter moving away: moon hidden longer Jupiter moving nearer: moon hidden shorter Difference indicates time for light to get to Earth

Michelson’s experiment
Spin the octagonal mirror, emit light pulses Change the speed at which the mirror spins At a certain rate, you’ll be able to see the light pulses At that rate, the mirror is turning 1/8 of a rotation in the time the light takes to go 2*35km Calculate the speed of light

Speed of Light 300,000,000 m/s in a vacuum (3 x 108 m/s)
We use the symbol c = 3 x 108 Slower, but about the same for air Used for ALL EM Waves

Using the Speed of Light
Just like with sound, if we know how fast something is traveling, we can use it to determine distance For light, velocity is c, so c = d/t

Light Years A light year is a unit of distance
It’s the distance light travels in one year 1 l.y. = 3 x 108 m/s x 3.1 x 108 sec/yr = 9.43 x 1016 m 2006 1 light year 2007

Electromagnetic Waves and Light
EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light

Types of EM Waves Radio waves Microwaves Infrared Visible Light
Ultraviolet X-rays Gamma Rays These waves are all EM waves, all travel at speed c The differ in their frequencies (and therefore, their wavelengths and energies)

EM Spectrum Gamma rays: radioactive decay
EM Waves are created by accelerating electric charge Gamma rays: radioactive decay Radio and microwaves: primarily produced by electronic equipment and antennas Infrared– heat! Produced by vibrating molecules Visible light: electrons changing state Spectrum Allocation Chart

Calculating Wavelength
In a vacuum, we know that EM waves travel at v = c = 3 x 108 The wave equation: v = f λ So we can relate frequency to wavelength For a radio wave with f = 2 x 107 Hz, λ = 3 x 108 / 2 x 107 = 15 m For green light with f = 6 x 1014 Hz, λ = 3 x 108 / 6 x 1014 = 5 x 10-7 m

Energy Remember: higher frequency, higher energy
So gamma rays have incredible energy Radio waves much, much less

Key Skills Know c Use c to relate distance travelled and time
Use wave equation to relate frequency and wavelength