Option G – Electromagnetic waves

Lesson 1

• Outline the nature of electromagnetic (EM) waves.

• Describe the different regions of the electromagnetic spectrum.

• Describe what is meant by the dispersion of EM waves.

• Describe the dispersion of EM waves in terms of the dependence of refractive index on wavelength.

Nature of electromagnetic waves

Nature of electromagnetic waves

• An oscillating electric charge produces varying electric and magnetic fields.

• Electromagnetic waves are transverse waves and all have the same speed in a vacuum.

Electromagnetic spectrum

λ ≈ 700 - 420 nm

λ ≈ 10-7 - 10-8 m

λ ≈ 10-9 - 10-11 m

λ ≈ 10-12 - 10-14 m

λ ≈ 10-4 - 10-6 m

λ ≈ 10-2 - 10-3 m

λ ≈ 10-1 - 103 m

Source of electromagnetic waves?

• You have 15 minutes to take notes from pages 600 and 601 about how different EM waves are produced. http://phet.colorado.edu/sims/radio-waves/radio-waves_en.jar

Type Source

Radio

Microwaves

Infra-red

Visible

Ultraviolet

X Ray

Gamma

Sources of EM radiation

Type Source

Radio Vibrating electrons e.g. AC current

Microwaves Excited semiconductors or vibrating electrons

Infra-red Electrons transitions between energy levels

Visible Electrons transitions between energy levels

Ultraviolet Electrons transitions between energy levels

X Ray Emitted when decelerate rapidly electrons e.g. when they hit a metal target

Gamma Emitted by nuclei after a nuclear reaction

Dispersion

Let’s make rainbows!

A reminder - Snell’s law

speed in substance 1 sinθ1

speed in substance 2 sinθ2=

A reminder - Snell’s law

In the case of light only, we usually define a quantity called the index of refraction for a given medium as

n = ccm

where c is the speed of light in a vacuum and cm is the speed of light in the medium

vacuum

c

cm

Dispersion

• This occurs because the index of refraction of the glass changes with wavelength. The index of refraction is slightly smaller for red light than it is for blue light, so the red light gets bent less.

Dispersion

Just one question!Page 605 Question 3