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

2. . 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. .

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

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

14. EM Spectrum
When we talk about light, we talk about this tiny slice of the spectrum
Spectrum Allocation Chart

15. 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)

16. 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

17. 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

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

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