Transcript electromagnetic

ELECTROMAGNETIC WAVES
SECONDARY 3 PHYSICS
WHAT ARE EM WAVES?
Electromagnetic waves (EM waves for short)
are waves that can travel in a vacuum.
These waves are created by the vibration of
an electric charge.
EM radiation is a wave that is produced as
follows:
When charges accelerate, they produce a
changing magnetic field.
This changing magnetic field creates a changing
electric field at 90° to it.
The electric field now causes a changing
magnetic field at right angles to it.
The magnetic and electric fields are able to
generate each other without any decrease in
strength if the fields move at 3  108 m.s-1.
Electromagnetic Spectrum
It is a group of different electromagnetic
waves.
There are 7 components in the spectrum.
Electromagnetic Spectrum
Gamma rays
X – rays
Ultraviolet
Visible light
Infra-red
Microwaves
Radio waves
Highest frequency
Shortest wavelength
Lowest frequency
Longest wavelength
Uses of Gamma Rays
Kill cancer cells
Study the nucleus
in atoms
Uses of X-rays
Take ‘pictures’ of
bones in the body
Study the crystal
structure of
crystalline
substances
Check for cracks in
metal plates
Uses of Ultraviolet
Detect counterfeit
notes
Gives the clothes
a ‘glow’ effect in
discotheques
Sun-tanning
Sterilise medical
equipment
Uses of Visible Light
Enable us to see
things
Photosynthesis in
plants
Uses of Infra-red
Heating
Haze
photography
Uses of Microwaves
Radar communication
Analysis of the
molecular and atomic
structure
Telephone
communications
Uses of Radio Waves
Radar
communications
TV and radio
broadcasting
Common Properties of EM Waves
All transverse waves
All travel at the speed of light. (3 x 108 m/s)
Can travel through solid, liquid, gas and
vacuum
Obey the laws of reflection and refraction
All can be absorbed and emitted by matter
The wave equation is applicable to all
Penetrating ability of electromagnetic radiation
The ability of EM radiation to go through (penetrate)
bone, glass or concrete depends on the energy of the
radiation and then also on the frequency of the
radiation.
Radio waves have the lowest frequency of EM
radiation and thereby the lowest energy. They are not
able to travel through the ground, therefore we cannot
receive a radio signal underground. The radio waves
are strongly diffracted (bent) around objects, while
the shorter radio waves are reflected by the charged
upper atmosphere fluctuate because of the changing
reflecting ability of this layer.
Microwaves have a low penetrative ability.
Cell phone masts that use microwaves must
have no obstructions in between them.
Infrared radiation has low energy. It is able to
warm the skin but cannot penetrate through the
skin.
Ultraviolet radiation has a frequency higher
than violet light in the visible spectrum. The
energy is such that it is harmful to the eyes and
can cause the skin to tan.
X-rays have a high energy and a high
penetrative ability. They can penetrate the
soft tissue in the body but cannot go
through the bone.
Gamma rays can pass through a few
centimetres of lead or concrete. They have a
large amount of energy
Energy carried by electromagnetic radiation
Energy  frequency
We calculate the energy of the radiation using:
E=hf
Where E is the energy of the radiation measured in
Joules (J)
h is plancks constant and has a value of
6,63  10-34 Js
f is the frequency of the radiation measured in
hertz (Hz)
Since f = c/ 
To calculate the energy of the photon, we use
the following equation:
E = hc/ 
Where E = Energy in joules (J)
h = planks constant (6,6  10-34 J.s)
f = frequency in hertz (Hz)
 = wavelength in metres (m)
c = speed of light (3  108 m.s-1)