Goal: To understand electromagnetic waves.

• Objectives:

• 1) to learn that the term electromagnetic wave is scarier sounding than it actually is

• 2) to understand how accelerating charges emit light

• 3) To learn about the electromagnetic spectrum

• 4) To understand the speed of light

Electromagnetic waves

• Are just a fancy way of saying light waves.

• However, this “light” isn’t just the light that we can see.

• It turns out that light comes in a lot of forms.

Properties of light

• Light can act like either a particle or a wave. Most commonly acts like a wave.

• Like waves you are familiar with, light waves have a wavelength and energy.

• Energy ~ speed of light / wavelength

Which of the following would emit light waves?

• Dentist X-ray machine

• An outgoing radio tower

• Radar machine

• The Earth

• Neon sign

• All of the above

• None of these

Spectrum of Electromagnetic Radiation

Region Wavelength(Angstroms)

Wavelength(centimeters)

Frequency(Hz)

Energy(eV)

Radio > 109 > 10 < 3 x 109 < 10-5

Microwave 109 - 106 10 - 0.01 3 x 109 - 3 x 1012 10-5 - 0.01

Infrared 106 - 7000 0.01 - 7 x 10-5 3 x 1012 - 4.3 x 1014 0.01 - 2

Visible 7000 - 4000 7 x 10-5 - 4 x 10-5 4.3 x 1014 - 7.5 x 1014 2 - 3

Ultraviolet 4000 - 10 4 x 10-5 - 10-7 7.5 x 1014 - 3 x 1017 3 - 103

X-Rays 10 - 0.1 10-7 - 10-9 3 x 1017 - 3 x 1019 103 - 105

Gamma Rays < 0.1 < 10-9 > 3 x 1019 > 105

Uses

• Different forms of light have different amounts of energy.

• This allows us to do different things.• For example, higher energy light waves go

further into our bodies.• Thus the use for X-rays.• Also, different substances reflect and

absorb light differently.• Lasers…

How is light created?

• Light is created by charges changing energy states – to conserve energy.

• Most comes from an atom.

• Later in the course we will look at atoms.

• When a charge changes directions, it changes energy.

• But energy is conserved!

• So, the particle will emit a PHOTON of light.

• Photon is a single wave of light.

The speed of light

• As we will learn later the speed of light cannot be broken directly.

• In a vacuum the speed of light (denoted as c) is:

• c = 1 / (ε0μ0)1/2 = 3 * 108 m/s

• OR: c = 1 light year / year

• (1 light year being the distance light travels in a year, and 1 light year = 6 trillion miles)

Conclusion

• We learn that light is a wave that can sometimes act like a particle.

• We learned about the different forms of light and what they are used for.

• We learned how and why light is created.• We learned about the speed of light in a

vacuum.

• After the test we will see what happens when we slow down light somehow or we bend or reflect it.