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To start with you should know what a spectrum is: when white
light is shone through a prism it is separated out into all the colours of the rainbow;
this is the visible spectrum. So white light is a mixture of all colours. Black is NOT a
colour; it is what you get when all the light is taken away.
That was easy. Some physicists pretend that light consists of
tiny particles which they call photons. They travel at the speed of light (what a
surprise). The speed of light is about 300,000,000 meters per second. When they hit
something they might bounce off, go right through or get absorbed. What happens depends a
bit on how much energy they have. If they bounce off something and then go into your eye
you will "see" the thing they have bounced off. Some things like glass and
perspex will let them go through; these materials are transparent. Black objects absorb
the photons so you should not be able to see black things: you will have to think about
this one. These poor old physicists get a little bit confused when they try to explain why
some photons go through a leaf, some are reflected, and some are absorbed. They say that
it is because they have different amounts of energy.
Other physicists pretend that light is made of waves. These
physicists measure the length of the waves and this helps them to explain what happens
when light hits leaves. The light with the longest wavelength (red) is absorbed by the
green stuff (chlorophyll) in the leaves. So is the light with the shortest wavelength
(blue). In between these two colours there is green light, this is allowed to pass right
through or is reflected. (Indigo and violet have shorter wavelengths than blue light.)
Well it is easy to explain some of the properties of light by
pretending that it is made of tiny particles called photons and it is easy to explain
other properties of light by pretending that it is some kind of wave.
This page is all about electromagnetic waves. The visible
spectrum is just one small part of the electromagnetic spectrum. These electromagnetic
waves are made up of two parts. The first part is an electric field. The second part is a
magnetic field. So that is why they are called electromagnetic waves. The two fields are
at right angles to each other.
I like these waves; they are used for radio broadcasts,
amateur radio, television, and mobile phones. Different parts of the radio spectrum have
been allocated to the various services. Radio waves have a much longer wavelength that
light waves. The longest waves are several kilometers in length. The shortest ones are
only millimeters long. (more to come when I have the
time.) Here is a bit more.....
The really nice thing about radio waves is that they
will make the electrons in a piece of copper wire move; this means that they generate
electric currents in the wire. In fact it works both ways: alternating currents in a
copper wire generate electromagnetic waves, and electromagnetic waves generate alternating
currents. The electric currents at "radio frequencies" (rf) are used by radio
and television transmitters and receivers. I use frequencies of 145MHz (VHF), 14MHz (20m
HF) and 3.7MHz (80m HF).
My HF radio transceiver can operate on all the HF
amateur bands, but 20m and 80m are my favorites at present. On the 80m band the
transceiver produces rf between 3.5MHz and 3.9MHz, the part of the transceiver which does
this is called an oscillator. I usually start with 3.77MHz but I can change the frequency
with the tuning knob. When I transmit, the transceiver "modulates" the rf with
an alternating current generated by my voice in a microphone. The modulating frequency is
called "af" or audio frequency. Someone listening to me uses a receiver which
can "demodulate" the radio signal. This means that the receiver removes the rf
to leave the af. This audio signal is fed into a loudspeaker. To find more about amateur
radio you should visit my Amateur
Radio Web.
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Microwaves have such a short wavelength that they are
very easily absorbed by water. This is why they are used in microwave ovens. What happens
is that when the water in your TV dinner absorbs the microwaves, the energy of the
microwaves is converted into heat: it makes the water molecules vibrate faster. Some
people are frightened that the radio waves coming out of their mobile phones are short
enough to cook their brains.
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These radio/light waves have a very short wavelength;
their wavelength is longer than visible light. Infra-red can be detected by special
infra-red film. If the police or MI5 shine an infra-red light on you they will be able to
take a picture of you using infra-red film: you will not know that they have taken your
photo. You have been warned!!! Animals like the pit-viper have infra-red detectors so that
they can find their prey in the dark. You have been warned again!!!
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The Visible Spectrum: Richard Of York Gave Battle In Vain. =
Red, Orange, Yellow, Green, Blue, Indigo, and Violet. are the colours of the visible
spectrum. We cannot see Infra-red, but we can feel it warm our skin when we sit in the
sun. Infra-red has a longer wavelength (less energy) than Red light. We cannot see
Ultra-violet light, but we feel our skin has been burnt by the sun if we were in the sun
too long yesterday. It is the Ultra-violet which is thought to cause skin cancer. UV light
has a shorter wavelength (more energy) than visible light. |
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These waves have very high energy and very short wave
lengths; shorter than visible light. Some animals like honey bees can see ultra-violet
light. Some plants have white flowers, at least you think that they are all white, but
they may appear to be different colours to a honey bee because of the amounts of
ultra-violet light which they reflect.
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X-Rays: |
X-Rays have so much energy and such a short wavelength
that they can go right through you. However, they cannot get through bone as easily as
they can get through muscle. This is because your bones contain so much Calcium. If you
have never had an X-Ray, try jumping off a high wall and breaking a bone. The doctors will
soon have you fixed: they take an X-Ray to see which bones have been broken. If you break
the bone in enough places they will have to use steel bolts to fix you up.
X-Ray can also be used to find other problems in your
body. If the doctors want to look for ulcers in your guts, they can give you a Barium
meal. Like Calcium, the Barium absorbs X-Rays so the doctors can look at parts of your
guts and find your ulcers. You get ulcers by worrying about your exams; so don't worry;
just do the revision.
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These are nasty ones. They have very high energy and
will even go through metals. So they can be used for finding tiny cracks in metals. You
cannot see the hairline cracks in an aeroplane wing with the naked eye. How would you like
the wings to fall off the plane on your summer holiday flight to Neasden or wherever it is
that you go. If the plane has been thoroughly checked you should be safe. Some radioactive
materials produce gamma rays. Gamma rays and X-Rays can cause cancer, but gamma rays can
also be used to destroy cancer cells: this is radio-therapy.
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