Waves



We have already mentioned the electromagnetic spectrum and been introduced to a few of the waves from within it. The complete spectrum contains many key areas ranging from radio to gamma. Remember that the speed of light is constant so as frequency gets larger, wavelength must get shorter and vice versa.



Visible light is the easiest place to start because, well, we can see it! Starting with red, we move towards blue and the wavelength gets shorter and the frequency gets longer. If you carry on past violet, we get ultra violet which is not visible but is still there. Likewise, before red is infrared which which is given out by all objects, also known as thermal radiation.

Waves are used extensively in communications. Beginning with the obvious choice which is radio waves. There are different radio waves depending on the band selected and as it will say on your radio set, it truly is short or long wave length and variations in between but all from the radio wave section of the spectrum. Mobile phones use a different type of wave- Microwaves. These are thought by some to cause brain abnormalities and other illnesses. Optical fibres have a type of light shone down them (total internal reflection) that sends signals. Either visible light or infrared can be sent along each fibre to transmit information.


The Doppler effect is observed when a body is emitting waves of a constant wavelength but either you or the body are moving. This causes the waves to appear stretched or compressed. This is why when an ambulance is driving towards you its pitch seems to change dramatically. It is the same with a distant galaxy, it is moving away from us, the waves it emits are "stretched" and the increase in wavelength is closer to red than before so we call it "Red Shift". Likewise, if moving towards us, it would be bluer than before. As all galaxies that we observe appear very slightly redder in their spectrum than they are, we can assume that they are all moving away from us and so we believe that the universe is still expanding.

When you first think of waves, you probably think of the beach and assume that you are not being very scientific.

Light reflection

Actually, you are. The waves of the sea link to most things that we will go over.

Longitudinal and transverse waves

Waves transmit energy or information from one place to another. Here there are 2 types of wave. The first is a longitudinal wave and is best demonstrated by pushing a slinky towards its end then pulling it back to show what looks like a darker patch rushing towards the other end. This is an area of compressed slinky moving. The second type of wave is best shown when winding up the cable for the vacuum cleaner, hold the plug and lift it up then flick it back down, this sends a high point or peak rushing towards the vacuum cleaner as you can see in the diagram.
The key terms are shown on the diagram;

Compression: where particles are bunched up together,
Rarefaction: where particles are pulled further apart,
Trough: lowest point,
Crest: highest point,
Wavelength: distance between two crests or two troughs,
Amplitude: height from centre axis to the crest or the tough.

Mechanical waves travel through a medium (particles) like water with the waves on the sea or a lake, they can be either longitudinal or transverse.

Refraction through different mediums

Electromagnetic waves such as light and microwaves are always transverse and can travel through a vacuum so don't need particles to transfer the energy. This links back to infrared heat coming from the Sun through a vacuum. These waves are oscillations in electric and magnetic fields.

Using the above diagram and the definitions, we can calculate things about the waves. The frequency is the number of complete waves completed in one second and the units are hertz (Hz). Finally, we can calculate the speed of a wave using the formula:

Speed (in meters per second) = frequency (Hz) x wavelength (m)

Also written as: v=fλ where the Greek letter lamda represents wavelength in meters
Waves react in three main waves when interacting with boundaries or surfaces.

In a reflection, light reflects off a surface and we see an image in that surface. Here, we can see that the normal is drawn at 90° to the mirror. It is crucial that the angle of incidence is equal to the angle of reflection.

Diffraction grate

An image in a plane mirror, looks upright, the same size and roughly the same distance behind the mirror as the object is in front of the mirror.
Real images can be formed on a screen.
Virtual images cannot be formed on a screen.



Different sound waves, loudness and pitch

If light passes from a low density medium like air, into a higher density medium like glass or water, it actually slows down very very slightly. As it is slowing slightly, it bends towards the normal at the boundary in the diagram on the right so rather than go straight through it bends.

Likewise, on leaving the glass block, it bends away from the normal as it pick up speed again.

If the two sides of the block are parallel, the the two rays of light (arriving and leaving) will also be parallel.


This last effect show what happens when parallel waves arrive at a narrow opening. Only certain parts of the waves can get through and the parts that do fan out in this pattern. This effect is called diffraction. It is not only on mechanical waves like water, electromagnetic waves are diffracted such as when TV signals are narrowed by hills, they warp and diffract.


Sound waves pass through air, solids, water...basically, anything with particles. It needs a medium! If you put a ringing bell into a jar, you can still hear it but if you suck out all of the air, you won't be able to hear it as there is nothing to carry the sound energy to the glass so it can pass it onto the air outside the jar.

There are sounds that we cannot hear like ultra sound or dog whistles, this is because the frequency of these waves falls outside our normal hearing which is between 20 Hz and 20000 Hz. Sound waves can be reflected, refracted and diffracted. These cause echoes and changes in sounds when they pass through different materials.

Pitch is a commonly used musical term and the way that it links to waves is quite simply demonstrated in these diagrams. Quiet sounds have a small amplitude and loud ones have a large amplitude. Low pitch sounds have a long wavelength and high pitch sounds have a short wavelength.


Key words and terms for this topic: gamma, x-ray, ultraviolet radiation, electromagnetic spectrum, wave speed, visible light, microwave, radio wave, band, optical fibre, Doppler effect, red-shift, blue-shift, Big Bang Theory, cosmic microwave background radiation (CMB), transverse, longitudinal, perpendicular, oscillation, compression, rarefaction, electromagnetic, mechanical wave, amplitude, wavelength, speed, frequency, plane mirror, incidence, reflection, real image, virtual image, boundary, refraction, diffraction, echo, sound wave, pitch, Doppler effect.

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