# Electromagnetic Waves - PowerPoint

Document Sample

```					                   Electromagnetic
Waves

11,16 April 2007        PHYS 202: Chapter 24   1
Introduction to Waves

A wave is a traveling disturbance and
carries energy from place to place

There are 2 basic types of waves:
Transverse waves
Longitudinal waves

11,16 April 2007           PHYS 202: Chapter 24   2
Transverse Waves

In a transverse wave, the disturbance
occurs perpendicular (transverse) to the
direction of wave
travel

Examples:
Waves in a string or rope
Electromagnetic waves
11,16 April 2007            PHYS 202: Chapter 24   3
Longitudinal Waves

In a longitudinal wave, the disturbance
occurs parallel to the direction of wave
travel

Examples:
Sound waves

11,16 April 2007           PHYS 202: Chapter 24   4
Anatomy of Vibration or Wave

Amplitude: distance
from midpoint to
crest, or to trough
Frequency f: number
of vibrations per unit
time
Period T: time for one complete vibration,
related to f by f = 1/T

Frequency is measured in hertz (Hz)
where 1 Hz = 1/s

11,16 April 2007              PHYS 202: Chapter 24   5
Wavelength & Wave Speed

The distance between any successive
identical parts of a wave is one
wavelength, λ

The speed v of a wave is the distance
covered each period, or       λ
v
T
Also         v=λf
11,16 April 2007           PHYS 202: Chapter 24            6
Creation of Electromagnetic Waves

Stationary charges create static
electric fields, and oscillating
charges create oscillating electric
fields
Charges moving with constant
speed create static magnetic
fields, and oscillating charges (or
currents) create oscillating
magnetic fields
Thus oscillating charges create
electromagnetic waves
each consisting of mutually
perpendicular and oscillating
electric & magnetic fields
11,16 April 2007               PHYS 202: Chapter 24   7
Electromagnetic Waves

Simulation

Electromagnetic (EM) waves
are transverse waves
can travel through a vacuum or a material
substance
all travel through a vacuum at the same
speed c = 3.00×108 m/s, called the speed
of light in a vacuum
11,16 April 2007            PHYS 202: Chapter 24                8
Electromagnetic Spectrum
The EM spectrum is the entire range of
wavelengths (or frequencies) of EM waves,
including the visible spectrum

Simulation
Example: Visible Light Waves

Find the range of wavelengths (in
nanometers) for visible light in the
frequency range between 4.0×1014 Hz
(red light) and 7.9×1014 Hz (violet
light)

11,16 April 2007              PHYS 202: Chapter 24   10
Visible Light

Wavelength in nanometers

Different wavelengths (or frequencies) of light
are perceived by the eye as different colors
Red colors have the largest wavelengths
(lowest frequencies), whereas blue and violet
colors have the smallest wavelengths (highest
frequencies)
White light is a combination of all the colors

11,16 April 2007           PHYS 202: Chapter 24         11
Energy Carried by Electromagnetic Waves

In 1865, Maxwell showed                          c2 = 1/(ε0 μ0)
and for EM waves             E=cB

Like other waves, EM waves carry energy
via their electric & magnetic fields

The total energy density u of an EM wave is its
total energy per unit volume
In a vacuum
u = ½ ε0 E2 + ½ B2/μ0 = ε0 E2 = B2/μ0

11,16 April 2007                PHYS 202: Chapter 24                    12
Intensity of Electromagnetic Waves

The intensity S of a wave is the power P it
carries perpendicularly through a surface
divided by the surface area A, or
P total energy
S              c u
A      t A

Thus
S = ½ c(ε0 E2+B2/μ0) = ε0 c E2 = c B2/μ0
11,16 April 2007                 PHYS 202: Chapter 24         13
Example: Energy Carried by EM Waves

On a cloudless day, the sunlight that
reaches the earth’s surface has an
average intensity of 1.0×103 W/m2.
What is the average EM energy
contained in 5.5 m3 of space just
above the earth’s surface?

11,16 April 2007                 PHYS 202: Chapter 24    14
Another Example: Energy Carried by EM Waves

The EM wave that delivers a cell-phone call
to a car has a magnetic field with an rms
value of 1.5×10–10 T. The wave passes
perpendicularly through an open window
having an area of 0.20 m2. How much
energy does this wave carry through the
window during a 45-s phone call? [P24.48]

11,16 April 2007        PHYS 202: Chapter 24        15
Doppler Effect for Sound

Waves from an approaching
source are bunched together
Waves from a receding source

The apparent change
in frequency of a wave
due to the relative motion of the
source and the observer is called
the Doppler effect

11,16 April 2007         PHYS 202: Chapter 24   16
Doppler Effect for Electromagnetic Waves

When EM waves in a vacuum travel along the
same line as the source and observer of the
waves, the Doppler effect is given by
    vrel 
fo  fs 1        if vrel  c
     c 
where
fo is the observed frequency
fs is the frequency emitted by the source
vrel is the relative speed of the observer and the
source
the + sign applies when the observer and
source approach each other
the – sign applies when they move apart
11,16 April 2007               PHYS 202: Chapter 24           17
Example: Doppler Effect for EM Waves

A speeder is pulling directly away and
increasing his distance from a police car
moving at 25 m/s relative to the ground.
The radar gun in the police car emits an
EM wave with a frequency of 7.0×109
Hz. The wave reflects from the speeder’s
car and returns to the police car, where
the frequency is measured to be 320 Hz
less than the emitted frequency. Find
the speeder’s speed relative to the
ground. [P24.48]

11,16 April 2007                 PHYS 202: Chapter 24     18
Another Example: Doppler Effect for EM Waves

The figure shows 3 situations A, B, and C in
which an observer and a source of EM waves
are moving along the same line. In each case,
the source emits a wave of the same
frequency. The arrows denote velocity vectors
relative to the ground. Rank the frequencies of
the observed waves in ascending order.

11,16 April 2007         PHYS 202: Chapter 24       19
Polarization
When the vibrations of a transverse wave
always occur along only one
direction, the wave is said to
be linearly polarized
This direction is called
direction of polarization
A linearly polarized wave
cannot pass through a slit
that is perpendicular to the
direction of polarization of
the wave
The idea of polarization has
no meaning for longitudinal
waves
11,16 April 2007                PHYS 202: Chapter 24   20
Linearly Polarized & Unpolarized EM Waves

In an EM wave that is linearly polarized, the
electric field oscillates along only one
direction, and the magnetic field also oscillates
along only one direction which
is perpendicular to the direction
of the electric field
In an unpolarized EM wave,
such as the light from the sun
or from an incandescent bulb,
the direction of polarization
does not remain fixed, but
fluctuates randomly in time

11,16 April 2007           PHYS 202: Chapter 24        21
Polarizing Materials
Linearly polarized light can be produced from
unpolarized light with the aid of a polarizing
material, which allows only the component of
the electric field along one direction to pass
through
while absorbing the field
component perpendicular
to this direction
The polarization direction
that a polarizing material
allows through is called
the transmission axis
The intensity of the transmitted polarized light
is a half that of the incident unpolarized light
11,16 April 2007              PHYS 202: Chapter 24       22
Malus’ Law
When 2 pieces of polarizing material are used
one after the other, the 1st is called the
polarizer and the 2nd the analyzer
If the average intensity of the polarized light
entering the analyzer is S0 , the average
intensity of the light leaving the analyzer is
S  S0 cos2θ
θ being the angle
between the
transmission axes
of the polarizer
and analyzer
This formula is called Malus’ law
11,16 April 2007              PHYS 202: Chapter 24      23
Crossed Polarizer & Analyzer

An example of polarizing material is Polaroid
plastic
When Polaroid sunglasses are uncrossed, the
transmitted light is dimmed due to the extra
thickness of the tinted plastic
When the sunglasses are crossed, the
intensity of the transmitted light is reduced to
zero, corresponding to θ = 90°

Simulation
Example: Malus’ Law

For each of the 3 sheets of polarizing
material shown in the figure, the
orientation of the transmission axis is
labeled relative to the vertical. The incident
beam of light is unpolarized and has an
intensity of 1260 W/m2. Determine the
intensity of the beam transmitted through
the 3 sheets when θ1 = 19.0°, θ2 = 55.0°,
and θ3 = 100.0°. [P24.36]

11,16 April 2007         PHYS 202: Chapter 24          25
Polarization by Reflection

Unpolarized light incident on a surface
and reflected by it becomes partially
polarized in a direction parallel to the
surface
When worn in the usual way, Polaroid
sunglasses each have a transmission
axis oriented vertically
They reduce glare by blocking
horizontally polarized light

Simulation
Questions
SAT 24.1.1

SAT 24.1.3

SAT 24.1.5

SAT 24.1.6

SAT 24.1.8

SAT 24.1.9

SAT 24.1.10

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 808 posted: 10/28/2010 language: English pages: 27