Electromagnetic
wave
(EM wave)
Electromagnetic wave (EM wave)
or electromagnetic radiation is a wave that is
created as a result of vibrations between an electric
field and magnetic field
A changing electric field produces magnetic field
and produced around a vibrating charge.
And according to Michael Faraday, the changing
magnetic field will produce electric field.
A wave carries energy as it propagates.
EM waves can travel through anything even on
vacuum, meaning they do not need any medium to
travel. They travel in a vacuum at a speed of 3x108
m/s.
ELECTROMAGNETIC SPECTRUM
EM waves are arranged in a gradual
progression from the waves of lowest
frequency to highest frequency. This
arrangement of EM waves is called
electromagnetic spectrum.
How do moving charges create magnetic
fields?
◦Any moving electric charge is surrounded
by an electric field and a magnetic field.
What happens when electric and magnetic
fields change?
◦A changing magnetic field creates a
changing electric field.
◦One example of this is a transformer which
transfers electric energy from one circuit to
another circuit.
◦In the main coil changing electric current
produces a changing magnetic field
◦Which then creates a changing electric field in
another coil producing an electric current
◦The reverse is also true.
Making Electromagnetic Waves
◦When an electric charge vibrates, the
electric field around it changes creating a
changing magnetic field.
Wavelengths decrease going along the EM spectrum from radio
waves to gamma rays.
Frequencies increase going along the EM spectrum from
radio waves to gamma rays.
Radio wave photons have the lowest
frequency and the least energy, and gamma
ray photons have the highest frequency and
the most energy.
The Electromagnetic Spectrum
◦Whenever radiation is
absorbed by matter,
photons transfer their
energy to the matter.
The Electromagnetic Spectrum
Before ionisation
After ionisation
electro
n
photon
Atom or
molecule
Chang
ed
atom or
molecul
e
The photon hits the atom or molecule,
and removes an electron.
Some high energy EM
radiation (ultraviolet, X-rays
and gamma rays) are known as
ionising radiation because they
have enough energy to remove
an electron from an atom or
molecule)
If cells are exposed to ionising radiation, they can damage the DNA in the nucleus of the cell.
This can cause mutations, and the cells divide constantly without control – this is cancer.
Very high doses of ionising radiation can kill cells.
Excessive exposure to UV radiation can lead to sunburn or even skin cancer.
Increased exposure = more damage
Practical Applications
Radio waves: Have the longest wavelengths
and the lowest frequencies; wavelengths range
from 1000s of meters to .001 m
Used in: RADAR, cooking food, satellite
transmissions
Microwaves
Microwaves are used to send signals between
mobile phones and mobile phone masts.
When you make calls on your mobile, your phone
emits microwave radiation. Some of this is
absorbed by your body and may cause heating of
body tissues.
This heating could result in medical conditions,
possibly including cancer, but there is no
conclusive evidence.
Precaution: limit the amount of time you spend
talking on a mobile phone!
Ultraviolet Light: Wavelengths range from 400 nm to 10 nm; the
frequency (and therefore the energy) is high enough with UV rays to
penetrate living cells and cause them damage.
Although we cannot see UV light, bees, bats, butterflies, some
small rodents and birds can.
UV on our skin produces vitamin D in our bodies. Too much UV can
lead to sunburn and skin cancer. UV rays are easily blocked by
clothing.
Used for sterilization because they kill bacteria.
X-Rays:
Used to look at solid structures, such
as bones and bridges (for cracks),
and for treatment of cancer.
◦X-rays are used by radiographers in hospitals
to check for broken bones.
◦X-rays pass easily through flesh, but are
absorbed by denser materials like bone and
metal.
◦X-ray imaging is also used in airports to check
the contents of bags.
◦Precautions: radiograhers wear lead aprons or
stand behind concrete to protect themselves.
Gamma Rays: Carry the most energy and have the shortest
wavelengths, less than one trillionth of a meter (10-12).
Gamma rays have enough energy to go through most materials easily; you
would need a 3-4 ft thick concrete wall to stop them!
Gamma rays are released by
nuclear reactions in nuclear power
plants, by nuclear bombs, and by
naturally occurring elements on
Earth.
Sometimes used in the treatment
of cancers.