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Revision-GCSE

Physics: Year 11

Now you get to laugh at how little I understand this stuff...

 

Wave Properties

There are two different types of waves, longitudinal and transverse. You need to know the differences between them (and how to spell them).

Transverse are the most common type of waves. For example: Light and other Electromagnetic (EM) waves and ripples on water.

In a transverse wave, the vibrations move at 90° from the direction of the wave.

Longitudinal waves are rarer. You can only find these as:

  • Sound waves
  • Shock waves (e.g. seismic waves)
  • A slinky plucked.
In longitudinal, the vibrations move in the same direction as the wave.

Where the wave is spread out is called rarefaction and where it's pushed together is called compression. See how these areas move in the animation.

A closer look...

When we draw out a wave, even if it's longitudinal, we always graph it as below:

 

You'll need to know what the following are: amplitude and wavelength.

  • Amplitude is the height of the wave from its undisturbed position (x-axis) to its peak. The bigger the amplitude, the more energy it carries. In sound, this means it'll be louder. In light, it means it'll be brighter.
  • Wavelength measures the length of the wave (funnily enough) -- the complete cycle of one wave.

It's very easy to make silly mistakes if asked to draw these, so study the following diagram:

Remember, take your time!

  • Frequency is the number of waves produced from a source (or passing a particular point) in one second. It is measured in Hertz. For example 1 Hz = 1 wave passing a point in a second.
  • Speed is simply frequency x wavelength.

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Reflection and Refraction

Both longitudinal and transverse waves can be reflected and refracted, but it's easier to see this in transverse.

Reflection

Reflection of light is all around us. It's what allows us to see anything (fortunately the section of the eye has been cut from the exam). Basically, objects absorb certain lights, while they reflect others. It's these reflected lights that give objects their colour.

Black is in reality, not a colour at all. It absorbs all coloured light, and so we just see a void.

If light hits a rough surface, the reflected light bouces all over the place. This is called diffused reflection. (And here's a pretty picture for you):

Notice that the incoming light it straight, and all in the same direction. The Normal as you know, is the imaginary line that is at 90° from the surface. Because the surface is uneven, the light hits it at different angles, so bounces back at different angles.

When the light hits a smooth, shiny surface, like a mirror, you get a better reflection:

All the reflected rays are in the same direction, meaning a better reflection.

A closer look at reflection

  • The image we see in a mirror is always the same distance behind the mirror, as the object is in front of it. It's always the same size as the object too.
  • Incidence rays are light waves bouncing from the object, and reflected rays are those that bounce back from the mirror.
  • The angle of incidence (between the normal and the incidence ray) is always equal to the angle of reflection.
  • The light given off from anything is called diverging rays (as they are spreading out).
  • Both longitudinal and transverse waves can be reflected (think echoes for sound).

Refraction

Like, you know how we're all massively interested in watching waves in water? Did you know if a wave passes from deep water, to shallow water, the waves are actually bent?

For some reason the syllabus has an obsession with water waves... Anyway, this bending of waves is called refraction. It happens in both longitudinal and transverse waves. The beautiful Ripple Tank picture shows you it...

When we convert this to real physicals, the rule is: when a wave travels from dense to less dense water, the wave is slowed down. Only part of the wave's speed is changed at first, and this causes it to bend.

When a wave enters a denser medium, it is slowed down and bends towards the normal.

When a wave enters a less dense medium, it speeds up and is bent away from the normal.

And, to show this, here's the old refraction with light and a glass block:

  • As usual, the normal is 90° from the object's surface.
  • As the light enters the dense glass, it bends towards the normal.
  • As the light leaves the glass to the less dense air, it bends away from the normal.
  • The refracted wave is called the emergent ray.

Remember though: If the wave moves from one density to another along the normal (at 90°) there's no refraction. This is because all the wave is being slowed at the same time. There's still a change in wavelength.

Depending on the object, there can also be reflection.

Internal Reflection

When you did the old refraction with glass test, you might have noticed that some of the light was reflected back, out of it. This is called internal reflection and happens when light is exiting something dense like glass, water or perspex.

When you change the angle of the light on the block, the reflected light varies. At a certain angle, called the critical angle all the light is reflected, and none is refracted. When this happens we have total internal reflection.

How's total internal reflection used?

In glass, the critical angle is around 42° so we can make binoculars, periscopes, etc. Prisms give us a better image for binoculars than mirrors would:

Optic fibres depend upon total internal reflection.The fibre is narrow and if it isn't bent too sharply, total internal reflection will always occur.

Optic fibres can be used to send electrical signals. They are better than using cables because the same about of wire can carry a lot more info, and they aren't effected by electronic signals.

Travelling long distances can cause the signals to become distorted though, just like wires.

Another use for optic fibres is in endoscopes. These have a lense system at each end. The optic fibres carry light down the endoscope, with is reflected from what you want to see, to the lense, and then back up. Endoscopes are used by doctors for looking inside people without chopping them up.

Diffraction (I think this is Double, but it's pretty easy anyway)

Ever wondered why you can hear someone talking around a corner? This is because waves spread out as they travel through something like a gap in a barrier. The wave has to "bend" to fit through the narrow gap, and this allows it to spread out. This spreading out is called diffusion. The next diagrams show how this works:

The gap is only a little smaller than the wave, so there's very little diffracion. Here, however, we have a much narrower gap, and so the wave is diffracted a lot more.

Naturally, diffraction depends on the wavelength. Small wavelengths are less likely to be affected by narrow gaps, because they'll fit through easily.

Large wavelengths, though, will be diffracted a lot, because they have to squeeze through everything.

If you live in the hills and you want to listen to the radio, you'll probably only be able to hear the long wavelength as it's easier for them to diffract over the hills, and to your house.

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Sound and Ultrasound

Sound, as you know is one of the few longitudinal wave. We hear sound because of the vibration of particles. This is the main difference between light and sound - sound waves cannot travel through a vaccuum.

As with all waves, sound waves can be:

The bigger the amplitude of a wave, the louder the sound. The greater the frequency, the higher the pitch.

There is only a certain range of frequencies that the human ear can detect. Anything with a higher frequency than this range is called ultra sound. It can be produced by electronic systems and is useful in many different ways:

  • Industria Cleaning - The waves vibrate and shake free dirt. This means delicate instruments can be cleaned without dismantling them. It's easy to precisely aim ultrasound, making it very effective.
  • Quality Control and Pre-natal Scanning - These two use the same principle. When ultrasound meets a boundary between two materials, they are partly reflected. The time taken for the reflections to reach a detector is a measure of how far away the boundary is. It allows us to produce a visual display, such as a foetus scan, or discover any flaws/cracks in industy.

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Light and Electromagnetic Waves

 

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All text copyright © 2006 to EJ Taylor. Page Template created by James Taylor. Site created: 10 April, 2006. Last revised: 2 August, 2015