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SPH4U1 – Waves – L7 – Electromagnetic Waves
Electromagnetic Waves & Light
1864 James Clerk Maxwell
 The distribution of electric charges, in space, produces an electric field.
 Magnetic field lines are continuous loops without beginning or end.
 Electric field lines, on the other hand, begin and end on electric charges.
 A changing electric field produces a magnetic field.
 A changing magnetic field produces an electric field.
CHARACTERISTICS of ELECTROMAGNETIC WAVES:
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Electromagnetic waves are produced whenever electric charges are accelerated.
The accelerated charge loses energy. This energy is carried away in the
electromagnetic wave.
If the electric charge is accelerated in periodic motion, the frequency of the
electromagnetic waves produced is exactly equal to the frequency of oscillation
of the charge.
All electromagnetic waves travel through a vacuum at a common speed (c= 3.0 x
108 m/s), and obey the universal wave equation, c= fλ
Electromagnetic waves consist of electric and magnetic fields oscillating in phase,
perpendicular to each other, and both at 90o to the direction of propagation of
the wave.
Electromagnetic waves exhibit the properties of interference, diffraction,
polarization, and refraction and can carry linear and angular momentum.
All electromagnetic waves can be polarized, and radiation where the electric field
vector is in only one plane is said to be plane-polarized.
SPH4U1 – Waves – L7 – Electromagnetic Waves
In 1887 Heinrich Hertz produced the first electromagnetic wave. He used a spark gap
and caused the electron to jump back and forth between the plates with a frequency of
1 MHz. An antenna (wire loop) was used to pick up the changing magnetic field. He
called them radio waves. From here on, a larger “spectrum” of electromagnetic waves
was established.
The Electromagnetic Spectrum
Type
Radio, Radar, TV
Range
104 - 1010 Hz
Microwaves
109 – 1012 Hz
Infrared
1011 – 4 x 1014 Hz
Visible light
4 x 1014 – 8 x 1014 Hz
Ultraviolet
8 x 1014 – 1017 Hz
X Rays
1015 – 1020 Hz
Gamma Rays
1019 – 1024 Hz
Cosmic Rays
>1024 Hz
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35
EX 1: Microwaves with a wavelength of 1.5 cm are used to transmit television signals
coast to coast, through a network of relay towers.
a)
What is the frequency of these microwaves?
b)
How long does it take to send the signal from Newfoundland to BC, a distance of
5000 km.
HMWK:
Pg. 533 #1-4
Pg. 534 #1-3
SPH4U1 – Waves – L7 – Electromagnetic Waves
ASSIGNMENT PROBLEMS
1. Light from a monochromatic source passes through a single slit with a width of 12.7
µm. If the fourth dark fringe appears at an angle of 10.0o, calculate the wavelength of
light used. What colour is this?
2. A helium–neon laser is used to produce light with a wavelength of 632.7 nm which is
passed through a single slit. If the adjacent minima are separated by a distance of 7.50
cm on a screen 2.50 m away, what is the width of the slit?
3. A single-slit interference pattern using blue light with a wavelength of 438 nm
creates a central maximum that is 6.50 cm wide. If the slit width used is 17.5 µm, how far
away is the screen?
4. A diffraction grating etched with 3500 lines/cm is used on a 685-nm wavelength light
source. At what angle is the third-order maximum located?
5. A picture frame manufacturer wishes to design picture frames that provide minimal
glare from the glass cover. In order to achieve this, a thin plastic film (nfilm = 1.35) is
placed on the glass surface (nglass = 1.52). If light reflected in the middle of the visible
spectrum with a wavelength of 550 nm is to be minimized, what film thickness is
required?
6. The thickness of a human hair is to be measured using the interference pattern
produced by an air wedge. Red light with a wavelength of 638 nm is used on an air
wedge that is 25.0 cm long. If 10 bright fringes are counted across 1.06 cm in the air
wedge, what is the thickness of the hair?
7. A soap film in air has a thickness of 175 nm. If the index of refraction of the soap film
is 1.35, what colour of light will be transmitted through? (i.e., What colour does the
bubble appear to be from the other side?)
8. The distance of closest approach between Saturn and Earth is approximately 1.21 x
109 km. How long, in minutes, does the sunlight reflected from the surface of Saturn
take to reach Earth at this point? (c = 3.00 x 108 m/s)