Electromagnetic Waves &
Interference of light
The discovery of electromagnetic waves, led us to the understanding of visible light (the light we
see in all the colors of the rainbow) as a type of electromagnetic wave. Other types of EM waves
exist, for example radio waves that you can use to listen to music in your car or to “look” at the far
reaches of the cosmos (top right). Infrared waves are emitted by warm blooded animals. A
“thermogram” (bottom right), which uses infrared radiation, can help us identify areas with
irregular temperature variations, as in the inflammation near the abdomen seen in the image.
Producing an electromagnetic wave

A traveling electromagnetic wave produced by
an ac generator attached to an antenna.
2
E and B field direction when V is positive.




Positive charges are in the top.
Current flows up.
RHR2 the B field is into the page.
In general:


E and B will be perpendicular to
each other.
They are both perpendicular to the
direction of propagation.



Point fingers along E
Curl fingers in the direction of B
Thumb will point in propagation
direction.
3
An electromagnetic wave.



Electric and Magnetic Fields are perpendicular.
Oscillating E field produces a B field and
Oscillating B field produces an E field…


Wave only needs the E and B fields, it can travel in
vacuum!
Speed of electromagnetic waves:
c
1
 0 0
 3.0 108 m / s
4
Light as an electromagnetic wave

c=constant


c=lf, so larger frequency means smaller
wavelength
Visible light wavelengths


Roses are 700nm, Violets are 400nm…
What are the frequencies?
5
Electromagnetic Spectrum

Radio waves



f=106-109 Hz
Alternating Current
(AC) circuits in
metal antennas
Molecules and
accelerated
electrons in space.
6
Electromagnetic Spectrum

Microwaves





f=109-1012 Hz
Long distance phone
conversations (cell
phones).
Cooking (absorption of
microwaves by water
molecules).
RADAR
Highest frequency
produces by purely
electronic devices.
7
Electromagnetic Spectrum

Infrared



f=1012-4.3x1014 Hz
Can feel as heat on
skin.
Some animals, such as
the pit viper, can detect
infrared light.


Allows to see warm
blooded animals in the
dark.
Emitted by warm
bodies: produced by
vibrations and rotations
of molecules.
8
Electromagnetic Spectrum

Visible




f=4.3x1014-7.5x1014 Hz
Light that we can see.
Each frequency is
interpreted by our
nervous system as a
different color.
Produced by electrons
changing their positions
within an atom

Quantum mechanics,
atomic level.
9
Electromagnetic Spectrum

Ultraviolet


f=7.5x1014-1017 Hz
Skin is sensitive to UV



Sun produces UV light,
Ozone in stratosphere
absorbs most of it




Sun tans (in low doses)
Skin cancer (in high doses)
Reducing ozone in the
stratosphere can have
unwelcome consequences!
Bees are sensitive to UV,
and flowers look different in
UV.
The flowers help the bees to
“bullseye” the pollen!
UV absorbing
chemicals in plants,
help bees and fend
off herbivores.
10
Electromagnetic Spectrum

X-rays





f=1017-1020 Hz
Generated by the
rapid deceleration of
high-speed
electrons.
Can pass through
soft tissue
Do not pass through
denser material,
such as bone.
Can used as “antismuggling”
diagnostic.
11
Electromagnetic Spectrum

g-rays



f>1020
Generated by rearrangement
of protons and neutrons in
nucleus of an atom.
Also, generated by collision of
particle of matter with particle
of antimatter:




Supernova explosions
Gamma-ray bursts



annihilation!
produces radiation, no
“particle” with mass is left.
Some of the most violent
events in the universe
STILL DON’T KNOW THEIR
SOURCE.
Highly penetrating and
destructive to cells.


Used to treat cancer
Used to kill microorganisms in
food.
12
Superposition
13
Interference: Two antennae

Point P0: midway
between the antennas



At point P1 the distance
l2 is greater than the
distance l1 by one
wavelength;


the waves travel the
same distance
they interfere
constructively.
P1 is also a point of
constructive
interference.
At Q1 the distance l2 is
greater than the
distance l1 by half a
wavelength

waves interfere
destructively
14
Young’s Two Slit Experiment


The first screen produces a
small source of light that
illuminates the two slits, S1
and S2.
After passing through these
slits the light spreads out
into an interference pattern

alternating bright and dark
fringes on a distant screen.
15
Two slits act as two circular sources


According to Huygens’s
principle, each of the two
slits in Young’s experiment
acts as a source of light
waves propagating outward
in all forward directions.
Light from the two sources
can overlap

Result: Interference pattern.
16
Pathlength from each slit to a screen



Light propagating
from two slits to a
distant screen along
parallel paths;
Paths make an angle
q relative to the
normal to the slits.
The difference in
path length: d sin q

d is the slit
separation.
17
Bright and dark fringes

Bright fringes


Path difference is a
“whole number” of
wavelengths.
Dark fringes

Path difference is a
“whole number plus a
half” wavelength.
18
Example 35.1 Two slit interference


Two slits with a
separation of 0.200
mm create an
interference pattern
on a screen 1.00 m
away.
If the third bright
fringe above the
central fringe is a
linear distance of 9.49
from it, what is the
wavelength of light
used in the
experiment?
19
Example 35.2 Broadcast Pattern



A radio station operates at a
frequency of 1500 KHz.
Two identical vertical dipole
antennae 400 m apart are used,
and they oscillate in phase.
For distances greater than 400 m,
in what direction is the intensity
greatest in the resulting radiation
pattern?
20