Chapter 23

PowerPoint Lectures for College Physics, Eighth Edition

Hugh D. Young and Robert M. Geller

Lectures by James L. Pazun

Chapter 23Chapter 23

Electromagnetic Waves

Goals for Chapter 24

• To understand electromagnetic waves, the speed of light, and the electromagnetic spectrum.

• To characterize sinusoidal waves and determine their energy.

• To describe the nature of light.

• To describe reflection, refraction and total internal reflection.

• To study polarization.

Radio, TV, light - electromagnetic waves abound!

See the caption on page 762.

Electromagnetic waves – Figures 23.1

Formed from an electric field and magnetic field orthonormal to each other, propagating at the speed of light (in a vacuum).

The electromagnetic wave – Figure 23.2

• The waves are transverse: electric to magnetic and both to the direction of propagation.

•The ratio of electric to magnetic magnitude is E=cB.

•The wave(s) travel in vacuum at c.

•Unlike other mechanical waves, there is no need for a medium to propagate.

Refer to Conceptual Analysis 23.1 and worked Example 23.1

The electromagnetic Spectrum – Figure 23.3

The spectrum runs from low energy. Low frequency,and large wavelength at left with radio and TV signals to high energy, high frequency and short wavelength at right with gamma rays.

The radiation determines what is seen – Figure 23.4

Different wavelengths reveal different objects.

Seeing in IR or UV reveals species-specific needs – Page 765

• Seeing in the UV, for example, steers insects to pollen that humans could not see.

Describing electromagnetic waves – Figure 23.5• The transverse waves of the electric and magnetic vectors move at the speed of light and may be cast in meaningful equations.

• Refer to Problem Solving Strategy 23.1, Example 23.2, and Example 23.3.

Energy is electromagnetic waves – Figure 23.7

• Go outside on a sunny day in a black t-shirt. You will soon realize that there is energy stored in electromagnetic radiation. In this case, infrared and visible.

• Refer to Examples 23.4 and 23.5.

Light can exert physical pressure – Figures 23.9, 23.10

• When present in large flux, photons can exert measurable force on objects.

• Massive photon flux from excimer lasers can slow molecules to a complete stop in a phenomenon called “laser cooling”.

Light manifests different properties – Figures 23.11,12

• The incandescent bulb projects a wide, incoherent spectrum of light. The surgical laser utilizes a coherent beam in a very narrow spectral window.

Propagation described as a wave front. – Figure 23.13

• Rather than drawing each wave, we treat all points on a wave front to be at the same point of variation at a given moment in time.

• The allows rays to approximate light behavior.

The ray approximation – Figure 23.14

• The approximation works for spherical and for planar waves.

• The branch of optics for which the ray optics approximation is valid is known as geometric optics.

• Chapter 26 will show physical optics, where the ray model breaks down.

Reflection and refraction – Figure 23.15

Reflection may be stated simply as “bounce back”. Refraction may be stated simply as “bend”.

Types of reflection – Figure 23.16

• If a surface is planar on the scale we observe, it will allow orderly (or specular) reflection. When you draw a line perpendicular to the flat surface, we can measure incoming and outgoing rays with respect to this 90o line, the normal.

• A surface that produces specular reflection is highly polished.

•Rough surfaces produce diffuse reflection.

•Refer to Conceptual Analysis 23.2.

Refraction - Figure 23.17

• Refer to the definitions in yellow on page 775 and use the index of refraction.

• Refer to the principle of geometric optics on pages 775-776.

Refraction (Figure 23.18), reflection also Figure (23.19)

To perform calculations, use the data in Table 23.1

Conceptual analyses 23.3, 23.4

The first analysis explores the properties of refracted waves while the second examines the specific example of movement from air to glass.

Workings in the eye – Figures 23.23 and 23.24

Refer to worked Example 23.7.

Refer to worked Example 23.8.

Total internal reflection – Figure23.25

Refraction is observed at shallow angles. As the angle of approach to the interface becomes more and more acute, there is a point where refraction ceases and only reflection is possible.

Total internal reflection II – Figures 23.27, 28

With the advent of modern lasers, fiber optics have become “light pipes”, sending signals over tremendous distances.

Polarization – Figures 23.32-35

If light could be thought of as a hoop, a polarizing material could be considered a picket-fence. Only hoops perfectly aligned get through.

Polarization II – Figures 23.36, 37

Two orthonormal polarizers can block all light.

Refer to Problem-Solving Strategy 23.3 and worked Example 23.8.

Chapter 23

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