P6 The Wave Model of Radiation

P6 for the 6's exam

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Waves- The Basics

Waves have amplitude, wavelength and frequency:
Amplitude- distance from rest position to crest.
Wavelenght- Length of full cycle of wave e.g. crest to crest.
Frequency- number of complete waves passing a point per second or number of waves produced each second. 1Hz = 1 wave per second.
Speed (m/s) = Frequency (Hz) x Wavelenght (m)
The speed of a wave is usually independent of the frequency or amplitude of the wave. Changes of speed are linked to the changes of wavelengh.

Transverse waves have sideways vibrations. Most waves are transverse:
- Light and all other EM waves
- Ripples on water
- A slinky spring wiggled up and down
In transverse waves the vibrations are at 90 to the direction of travel of the wave.
Longitudinal waves have vibrations along the same line:
- Sound waves and ultrasound
- Shock waves
-A slinky spring when you push the end
In longitudinal waves the vibrations are along the same direction as the wave is travelling.

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Wave Properties

All waves can be reflected, refracted and diffracted. When waves arrive at an obstacle (or meet new material) their direction of travel can be changed.

Waves can be reflected:
- When waves hit an boundary between one medium and another, they are partially reflected.
- The angle of reflection, r, of the waves is the same as the angle of incidence, i. These angles are always measured from the normal.
Refraction- waves change speed and direction.
- Waves travel different speeds in substances which have different densities. e.g. light wave travel slower in denser media. Sound waves travel faster in denser substances.
- If a wave hits a boundary face on, it slows but carries on in the same direction. But if a wave meets a different medium at an angle, it slows and changes direction- refracted.
When light shines on a glass window pane, most passes through and gets refracted:
- As light passes from air to glass, it slows down. This causes the light to refract towards the normal.
- When the light reaches the glass to air boundary on the other side of the window, it speeds up and bends away from the normal.

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Wave Properties Cont.

Water waves travel faster in deeper water, causing refraction when waves hit a boundary, between 2 depths, at an angle. There's a change in direction and a change in wavelength.

Diffraction- Waves spreading out at the edges when they pass through a gap or past an object. The narrower the gap, the longer the wavelength and the more the waves spread out (the waves become curved).

Total internal reflection happens above the critical angle.
If the angle of incidence (i) is....
....Less than the critical angle- most of the light passes out but a little bit of it is internally reflected.
....Equal to the critical angle- the emerging ray comes out along the surface. There's quite a bit of internal reflection.
....Greater than the critical angle- no light comes out, its all internally reflected. This is called total internal reflection and would only happen when the wave would speed up at the boundary. Optical fibres work by total internal reflection.

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Wave Interference

When waves meet they can cause disturbance. Water waves disturb water, sound waves disturb air etc.

When 2 waves meet they both cause their own disturbances and their effects combine- this is interference.

Constructive interference- when waves arrive 'in step' they disturb in the same direction and reinforce each other.



Destructive interference- where waves meet 'out of step', they disturb in opposite directions and cancel out.
The total amplitude of the waves at a point is the sum of the individual displacements at that point, taking direction into account.

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Wave Interference Cont.

Interference of light makes bright and dark bits. You can observe interference in a dark room by shining beams of light through narrow slits and you will see and interference pattern.

This is because:
- At certain points on the screen the waves will arrive 'in step' creating constructive interference- the amplitude of the wave is doubled= a bright band of light. This happens where the distance travelled by both waves is the same or different by a whole number of wavelengths.
- At other points the waves will arrive 'out of step' creating destructive interference = dark patches to occur. This happens at 1/2 wavelengths.

Light and sound must travel as waves. The fact that they can both diffract and interfere shows they have wave properties. If they were just tiny particles, they would just pass straight through a narrow gap, and if the gap was blocked, they'd either be absorbed, transmitted or reflected back.
If they were particles, there wouldnt be and positive or negative disturbances to interfere with eachother= no interference patterns.

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Electromagnetic Radiation

There are 7 types of Electromagnetic (EM) waves. EM radiation occurs at many different wavelengths. Waves with similar wavelengths tend to have similar properties.
From Radio Waves through to Gamma Rays there is increasing frequency.
From Gamma Rays through to Radio Waves there is increasing wavelength.
All EM waves travel at the same speed through space which is a vacuum. So waves with a shorter wavelength have a higher frequency. Each colour of visible light has a different frequency.

Sound waves cant travel through a vacuum. This is because they are transmitted by vibrating particles and there are no vibrating particles in space.

EM radiation doesn't just act like waves- it can also act like its made up of tiny particles. These particles are tiny packets of energy called photons.
The energy delivered by each photon in a beam of electromagnetic radiation depends on the frequency of the EM waves.
The higher the frequency, the higher the energy the photon delivers.
In a beam of EM radiation of a particular frequency, each photon carries the same amount of energy- this amount of energy depends on the frequency of the radiation. This means that the intensity of radiation beam depends on the number of photons that arrive each second and the energy of the photons in the beam

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Uses of EM Waves

Radio waves and Microwaves are good at transmitting information over long distances and therefore good for communication. This is because they don't get absorbed by the earths atmosphere as much as most waves in the middle of the EM spectrum.

TV and FM radio transmissions have very short wavelengths compared to most radio waves. Microwaves used for mobile phone communication have very long wavelengths compared to most microwaves.

Microwaves that can pass easily through the atmosphere are used for satillite communication.

For satellite TV, the signals from a transmitter are transmitted into space where it's absorbed by the satellite recovery dish. The satellite then transmits the signal back to earth in a different direction where its received by a satellite dish on the ground.
- The dishes are made out of metal that reflects the microwaves into the reciever rather than absorbing them.

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Uses of EM Waves Cont.

Microwave ovens use a different wavelength from satellites. The microwaves used in microwave ovens have a different wavelength. These microwaves are absorbed by the water molecules in the food.

X-rays are used to identify fractures. X-rays easily pass through flesh but not easily through denser material like bones. Its the varying amount of radiation that's absorbed that makes an x-ray image.

Infrared and light are used in optical fibres as the signal doesnt weaken too much as it travels along.

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Adding Information to Waves

Information is converted into signals and then sent long distances down telephone wires or carried on EM waves. There are 2 ways that you can send information as waves- AM and FM.

AM radio waves have varying amplitude.

An Am radio transmitter sends out a continuous carrier wave. The signal (e.g. music) is superimposed on the carrier wave using amplitude modulation.

AM- Amplitude modulation:
- The sound wave from the music 'modulates' or changes the carrier wave by changing its amplitude. The 2 waves combine so that the information is carried by the pattern of the final waves variation.

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Adding Information to Waves Cont.

FM radio waves have varying frequency. It's the same idea as AM but instead of changing the carrier waves amplitude, you change it's frequency.

FM- Frequency modulation
- Where there's a peak in the signal wave, the frequency of the modulated wave is decreased. Where there's a trough in the signal wave, the frequency is increased.

AM transmission has changing amplitude (but constant frequency).

FM transmission has changing frequency (but constant amplitude).

Receivers recover the original signal. They ignore the carrier part and extract the original signal.

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Analogue and Digital Signals

Analogue signals vary but Digital is either 'on' or 'off'.

The amplitude or frequency of an analogue signal vary continuously. An analogue signal can take any value in a particular range.




Digital signals can only take 2 values- they're made up of 'pulses'- on or off.




A digital receiver will decode these pulses to get a copy of the original signal.

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Analogue and Digital Signals Cont.

Signals have to be amplified. Both digital and analogue signals weaken as they travel, so they need to be amplified along their route. They also pick up interference or noise from electrical disturbances or other signals.

Digital signals are far better quality than analogue signals.

A 'noisy' digital signal is easy to 'clean up' as it's obvious what it's supposed to be- the noise doesn't get amplified.

But, if you recieve a 'noisy' analogue signal, it would be dificult to know what the original looked like. If you amplify a noisy analogue signal, you amplify the noise as well.

Digital is much better quality as the information received is the same as the original.
Advantages of Digital:
Easy to process using computers as they are digital too. 
Transmit several signals at once using just one cable or EM wave= so you can send more information than analogue at one time.

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