The Basics of Light
One thing that all
the forms of
electromagnetic
radiation have in
common is that
they can travel
through empty
space.
Electromagnetic Radiation
• Electromagnetic radiation or “light” is a
form of energy.
• Has both electric (E) and magnetic (H)
components.
• Characterized by:
–Wavelength (l)
–Amplitude (A)
Visible light waves are the only
electromagnetic waves we can see. We
see these waves as the colors of the
rainbow. Each color has a different
wavelength. Red has the longest
wavelength and violet has the shortest
wavelength. When all the waves are seen
together, they make white light.
The Particle/Wave Duality
 Light is a particle
 but it has no mass
 blue light can knock
electrons off metal plate
 red light can’t – has less
energy
 Light is a wave
 light goes out in all
directions
 light “bends” around
corners (like waves around
a pier)
 light reflects (mirrors)
 light refracts (bends)
INTERFERENCE of the light
Interference is the interaction between waves traveling in the same medium.
Electromagnetic Spectrum
Electromagnetic Radiation
(cont.)
• We classify electromagnetic radiation by
wavelength.
• Visible radiation takes up only a small part of
the electromagnetic spectrum.
The Electromagnetic Spectrum
Radio Waves
• Created by oscillations (vibrations) of electrons
in a conductor.
• Very long wavelength (few km to few cm) – low
frequency – so low energy waves.
• Used to transmit radio and TV signals.
• Travel in straight lines, but can be bounced off
the upper atmosphere, so can travel around the
world.
Radio
Radio waves are
very long
compared to waves
from
the rest of the
spectrum
Radio telescopes
use a large metal
dish to help detect
radio waves
The Electromagnetic Spectrum
MICROWAVES
• Wavelength from cm to .0001 m
• Used for cooking food
• Microwaves make water molecules vibrate, which causes
food to get very hot very quickly.
• Penetrate only a few cm, the heat is then carried inside by
conduction
MICROWAVES
• Microwaves are also used for communication.
• Mobile telephones use microwaves.
• They are also used to communicate with satellites as they can
pass through the upper atmosphere
• They also do not spread out much, so they are easy to use
with a dish.
The Electromagnetic Spectrum
INFRA-RED
• Everything gives out Infra-red
• This allows cameras to use IR instead of visible light for nightvision.
• They are also used for thermo-imaging cameras, to find
people buried in earthquakes. They will be being used in New
York.
• They are also used for remote controls
Infrared Radiation
Wavelength: 1 um – 1000 um
We emit IR light. In
fact all the objects
around us emit IR
radiation. Hotter
objects emit more
IR radiation, which
we are warmed by,
and which we
sense with our skin.
IR photo of a man
IR photo of the dust in the Solar System
The Electromagnetic Spectrum
Visible Light
White light is made up of light from all different colors
.
Intensity of Light
• Intensity is lowered when the light is spread out.
The Electromagnetic Spectrum
ULTRA VIOLET
• Shorter wavelength – 0.000001m to
0.000000001m, frequency higher.
• Carry more energy, and can penetrate top layers
of skin, damaging the lower layers
• This can cause sun burn, and possibly skin
cancer of a long period of time.
• The eyes are very sensitive to UV, so
sunglasses should always be worn in bright
sunshine.
• Most UV radiation from the sun is filtered by the
ozone layer.
Ultraviolet Light
Some birds & bees can see
UV as well as visible light.
Ultraviolet light has just the right
energy to break molecular bonds
apart. For this reason it is
detrimental to life. The Earth
has a natural shield to Solar UV
light in the form of an ozone
layer (80 km above the surface).
10% of sunlight is in the UV
The Electromagnetic Spectrum
X-Rays
• Their ability to pass through soft tissue,
but not bone, makes them very useful in
medicine.
• They are completely stopped by metals,
making them useful for security scanning
at airports.
X-Rays
• Because they carry a lot of energy they can
easily damage or kill cells.
• This can lead to cancer if over-exposure occurs.
• Very small amounts are used in hospitals and
are relatively safe, however the operator will
stand behind a lead screen to stop unnecessary
exposure
• X-rays are also used to treat cancer by killing
cancerous cells.
The Electromagnetic Spectrum
X-Rays and
Gamma Rays
•
X-rays and gamma rays absorbed in the Earth’s atmosphere
•
observatories must be sent into space
•
produced by matter heated to millions of degrees
•
caused by cosmic explosions, high speed collisions
X-rays
X-rays were discovered 1895
by Wilhelm Conrad Roentgen
(a German scientist) by
accident. A week later he took
this x-ray of his wife.
Gamma Rays for Medicine:
• Gamma rays are
used in medicine
to kill and treat
certain types of
tumors and
cancers.
•.
RISKS and BENEFITS
• X-Rays and Gamma rays are both used in
medicine.
• Their benefits are they destroy cancer,
and avoid surgery, as well as use for
diagnostic purposes.
• They can also cause cancer, and may
cause sickness if exposures are too
large.
THE COMPLETE
ELECTROMAGNETIC SPECTRUM
Long
WAVELENGTH
Short
Low
FREQUENCY
High
Low
ENERGY
High