Chapter 32

© 2012 Pearson Education, Inc.

{{Chapter 32Chapter 32

Electromagnetic Waves (cont.)Electromagnetic Waves (cont.)


Standing electromagnetic wavesStanding electromagnetic waves• Electromagnetic waves can be reflected by a Electromagnetic waves can be reflected by a

conductor or dielectric, which can lead to conductor or dielectric, which can lead to standing wavesstanding waves. (See Figure 32.22 below.). (See Figure 32.22 below.)

• Mathematically, standing waves are a Mathematically, standing waves are a superposition of incoming and outgoing superposition of incoming and outgoing waves.waves.


The drawing shows a sinusoidal electromagnetic standing wave. The average Poynting vector in this wave

Q32.8

A. points along the x-axis.

B. points along the y-axis.

C. points along the z-axis.

D. is zero.

E. none of the above


The drawing shows a sinusoidal electromagnetic standing wave. The average Poynting vector in this wave

A32.8

A. points along the x-axis.

B. points along the y-axis.

C. points along the z-axis.

D. is zero.

E. none of the above


{{Chapter 33Chapter 33

The Nature and Propagation of LightThe Nature and Propagation of Light


The nature of lightThe nature of light Light has properties of Light has properties of

bothboth waves and waves and particles. The wave particles. The wave model is easier for model is easier for explaining explaining propagation, but some propagation, but some other behavior other behavior requires the particle requires the particle model.model.

The The raysrays are are perpendicular to the perpendicular to the wave fronts (wave fronts (cross cross sections of the wave which sections of the wave which are in phaseare in phase)). .

This chapter will This chapter will concentrate on the ray concentrate on the ray perspectiveperspective


Reflection and refraction Reflection and refraction When light strikes a surface, it is (in general) both reflected and refracted.


Specular and diffuse reflectionSpecular and diffuse reflection• Specular reflectionSpecular reflection occurs at a very smooth surface (left occurs at a very smooth surface (left

figure). figure). • Diffuse reflectionDiffuse reflection occurs at a rough surface (right occurs at a rough surface (right

figure). figure). • Our primary concern is with specular reflection. Our primary concern is with specular reflection.


Laws of reflection and refractionLaws of reflection and refraction

• The The index of refraction index of refraction is is n = c/v >n = c/v >1.1.

• Angles are measured Angles are measured with respect to the with respect to the normalnormal..

• ReflectionReflection: The angle : The angle of reflection is equal to of reflection is equal to the angle of incidence.the angle of incidence.

• RefractionRefraction: Snell’s law : Snell’s law applies.applies.

• In a material In a material = = 00//nn..• Figure 33.7 (right) Figure 33.7 (right)

illustrates the laws of illustrates the laws of reflection and reflection and refraction.refraction.


Reflection and refraction in three casesReflection and refraction in three cases

• Figure 33.8 below shows three important cases:Figure 33.8 below shows three important cases: If If nnbb > n > naa, , the refracted ray is bent the refracted ray is bent towardtoward the normal. the normal. If If nnbb < n < naa, the refracted ray is bent , the refracted ray is bent away fromaway from the the

normal.normal. A ray oriented along the normal never bends.A ray oriented along the normal never bends.


Why does the ruler appear to be bent?Why does the ruler appear to be bent?

• The straight ruler in The straight ruler in Figure 33.9(a) appears to Figure 33.9(a) appears to bend at the surface of the bend at the surface of the water. water.

• Figure 33.9(b) shows why.Figure 33.9(b) shows why.


Some indexes of refractionSome indexes of refraction


Light passes from vacuum (index of refraction n = 1) into water (n = 1.333).

If the incident angle a is in the range 0° < a < 90°,

Q33.2

A. the refracted angle is greater than the incident angle.

B. the refracted angle is equal to the incident angle.

C. the refracted angle is less than the incident angle.

D. the answer depends on the specific value of a .


Light passes from vacuum (index of refraction n = 1) into water (n = 1.333).

If the incident angle a is in the range 0° < a < 90°,

A33.2

A. the refracted angle is greater than the incident angle.

B. the refracted angle is equal to the incident angle.

C. the refracted angle is less than the incident angle.

D. the answer depends on the specific value of a .


When light passes from vacuum (index of refraction n = 1) into water (n = 1.333),

Q33.1

A. the wavelength increases and the frequency is unchanged.

B. the wavelength decreases and the frequency is unchanged.

C. the wavelength is unchanged and the frequency increases.

D. the wavelength is unchanged and the frequency decreases.

E. both the wavelength and the frequency change.


When light passes from vacuum (index of refraction n = 1) into water (n = 1.333),

A33.1

A. the wavelength increases and the frequency is unchanged.

B. the wavelength decreases and the frequency is unchanged.

C. the wavelength is unchanged and the frequency increases.

D. the wavelength is unchanged and the frequency decreases.

E. both the wavelength and the frequency change.


Light passes from a medium of index of refraction na into a second medium of index of refraction nb. The angles of incidence and refraction are a and b, respectively.

If na < nb,

Q33.3

a > b and the light speeds up as it enters the second medium.

B. a > b and the light slows down as it enters the second medium.

C. a < b and the light speeds up as it enters the second medium.

D. a < b and the light slows down as it enters the second medium.


Light passes from a medium of index of refraction na into a second medium of index of refraction nb. The angles of incidence and refraction are a and b, respectively.

If na < nb,

A33.3

a > b and the light speeds up as it enters the second medium.

B. a > b and the light slows down as it enters the second medium.

C. a < b and the light speeds up as it enters the second medium.

D. a < b and the light slows down as it enters the second medium.


Total internal reflectionTotal internal reflection• Light striking at the Light striking at the critical anglecritical angle emerges tangent to emerges tangent to

the surface. (See Figure 33.13 below.)the surface. (See Figure 33.13 below.)• If If aa > > critcrit, the light is undergoes , the light is undergoes total internal total internal

reflectionreflection..


Some applications of total internal Some applications of total internal reflectionreflection

• A binocular using Porro prisms A binocular using Porro prisms (below) and a “light pipe” (right) make (below) and a “light pipe” (right) make use of total internal reflection in their use of total internal reflection in their design.design.


A diamond and a periscopeA diamond and a periscope

• Diamonds sparkle because they are cut so that Diamonds sparkle because they are cut so that total internal reflection occurs on their back total internal reflection occurs on their back surfaces. See Figure 33.17 below.surfaces. See Figure 33.17 below.


DispersionDispersion• DispersionDispersion: The index of : The index of

refraction depends on the refraction depends on the wavelength of the light. See wavelength of the light. See Figure 33.18 (right).Figure 33.18 (right).

• Figure 33.19 (below) shows Figure 33.19 (below) shows dispersion by a prism.dispersion by a prism.

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Chapter 32