P6- The wave model of radiation

• A summary of the P6- The wave model of radiation module from the OCR 21st century science
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• Created by: R_Hall
• Created on: 15-05-11 16:43

What is a wave?

• A wave is a disturbance moving through a material (a medium)
• The source of a wave is something that vibrates
• TRANVERSE WAVE- a wave where the particles of a medium move at right angles to the direction in which the wave moves (water waves)
• LONGITUDINAL WAVE- a wave where the particles vibrate in the same direction as the wave (imagine a slinky compressing)
• Sound is a longitudinal wave; a sound source vibrates, causing compression pulses in the air nearby
• Sound waves can travel through any gas, through liquids and solids
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Describing waves

• AMPLITUDE- The max. distance each point on a medium moves as a wave passes through it (in metres)
• FREQUENCY- The number waves that pass every point in the medium every second (in Hz- 1Hz= 1 wave per second)
• WAVE SPEED- The speed at which each wave crest passes through the medium (in m/s)
• WAVELENGTH- The length of a complete wave, from crest to crest or place to place (in metres)
• Amplitude and frequency depend on source, wave speed depends on medium
• Wave speed (m/s) = Frequency (Hz) x Wave length (m)
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Wave properties

• Waves do not slow down as they travel- the wave speed in constant.
• The wave loses amplitude as it travels due to friction
• Reflection- When a wave hits a barrier, it bounces back off it. The angle of reflection (r) = the angle of incidence (i)
• Refraction- The wavelength of a wave changes if they travel from a medium to another where their speed is different. If water waves go from a deep region to shallow, the wavelength is smaller
• Diffraction- When waves hit the edge of a barrier or pass through a gap, they bend a little and spread out into the region behind the barrier
• Interference-  If 2 waves of the same frequency meet, they merge and form an interference. Two waves that are "in step" produce a large disturbance, whereas 2 "out of step" waves cancel each other out
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Bending light beams

• Total internal reflection (TIR) is when a light beam crosses into a medium where it moves faster, it is bent towards the boundary of the media. If the wave hits the boundary at an angle greater than the critical angle, it is reflected.
• An optical fibre uses TIR. Light from an object enters a bundle of optical fibres and is reflected repeatedly until it comes out off the other end. The light can travel a long way without getting weaker as the reflection is total, and little light is lost through the sides
• A spectrum is formed when white light passes through a triangular prism and emerges as a series of colours.
• The splitting of white light into colours is called dispersion
• Red light has the lowest frequency (and shortest wavelength), violet light has the highest frequency (and the shortest wavelength)
• Dispersion happens because light of different frequencies travels through glass at different speeds.
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Electromagnetic waves

• Electromagnetic waves can travel through a vacuum
• They can all travel at a speed of 300,000 kilometres per second (the speed of light)
• They are all transverse waves- however it is tiny electrical and magnetic fields that vibrate rather than any material
• They transfer energy- a source loses energy when it is emitted, a material gains energy when it is absorbed
• PHOTON- A packet of electromagnetic radiation
• The higher the frequency of a elecmag. wave, the more energy the photon has
• The intensity of a beam of radiation depends on the energy of each photon and the number of photons being absorbed each second
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Wave properties

• Waves do not slow down as they travel- the wave speed in constant.
• The wave loses amplitude as it travels due to friction
• Reflection- When a wave hits a barrier, it bounces back off it. The angle of reflection (r) = the angle of incidence (i)
• Refraction- The wavelength of a wave changes if they travel from a medium to another where their speed is different. If water waves go from a deep region to shallow, the wavelength is smaller
• Diffraction- When waves hit the edge of a barrier or pass through a gap, they bend a little and spread out into the region behind the barrier
• Interference-  If 2 waves of the same frequency meet, they merge and form an interference. Two waves that are "in step" produce a large disturbance, whereas 2 "out of step" waves cancel each other out
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Above the visible

• Ultraviolet radiation is emitted by very hot objects (eg. the Sun)
• Most of the Sun's UV does not reach the Earth, the band of ozone gases in the atmosphere absorb UV with frequencies closer to visible light
• If too much UV is absorbed, it may cause skin cancer. Dark skin doesn't allow UV to penetrate deep into the skin, it is absorbed near the surface. Fair skin lets UV go deeper- a tan is developed to protect
• UV can damage the retinas, but produces Vit. D- immune system
• Uv can make some materials emit visible light and glow- fluoresce
• Ionizing Radiation- Radiation with photons of sufficient energy to remove electrons - damage living cells by altering or destroying
• High frequency X-Rays are vey penetrating. They pass through lead
• Low-frequency X-Rays are less so, these are used to X-ray photos. Exposure time must still be short as they are harmful
• Gamma rays come from radioactive materials and have short wavelengths
• Like X-rays, they can kill cancer cells and take bone photos
• They can kill bacteria- sterilizing medical equipment and food
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Below the visible

• Infrared- radiation can not be detected by the human
• When your skin absorbs infrared, it heats. All objects emit a wide range of infrared frequencies because of the movement of the atoms or molecules
• As something heats, it emits more infrared.
• Most solids absorb infrared (rising temp). High-frequency infrared can pass through transparent plastic and glass (longer wavelengths are absorbed of reflected). Greenhouse principle- light and high frequency infrared in, lower frequency reflected
• Microwaves have heating effect.
• Magnetron emits microwaves in microwave oven. It emits microwaves at a lower frequency than the frequency where water absorbs it best. If absorption was better, the microwaves would only heat the surface
• Microwaves have the highest frequencies (short wavelength) of all radio waves. Used for mobiles and sattelite TV as they are not absorbed by the ionsphere
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• Light is a type of radiation
• All radiation travels in all directions from a source, it may be reflected (bounce off), transmitted (go through) or be absorbed (transfer energy to a material).
• If light were a stream of tiny particles, it would not produce an interference pattern like it does. Also, light diffracts.
• Water and sound waves are identified as waves as they wave and vibrate when the pass through a medium, we can see the medium pulse.
• Light can travel through a vacuum; waves need a medium
• Light is an electromagnetic wave, it consists of constantly varying electric and magnetic fields that move together through space
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• The there is a.c. (alternating current) in a piece of wire, the wire acts as an aerial and produces radio waves. When the radio waves reach another aerial, they generate a.c. there.
• Audio frequency (AF)- the frequency (1kHz) of the incoming sound wave coming out of  the loud speaker of a radio system
• The RF oscillator creates the high frequency a.c. needed to produce the radio waves.The stream of waves is the carrier
• The AF signals are used to modulate (vary) the amplitude of the carrier so the height of the RF waves is a copy of the incoming sound waves
• Before the modulated wave is sent to the aerial, its amplitude is boosted by an amplifier
• In the receiving aerial, the incoming radio waves generate electrical signals
• The tuning circuit selects the frequency of one RF carrier and the demodulator removes the RF carrier
• Amplitude modulation- The amplitude of the RF is varied by the AF signal
• Frequency modulation- The frequency is varied instead of the amplitude
• FM is less effected by electrical interference- noise
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Going digital

• When sound waves enter a microphone, a varying voltage is generated- continuous variations are called analogue signals
• The varying voltage can be converted into digital signals (represented by numbers)
• Digital code-  a string of 1 and 0 that represents the analogue signal. At the receiving end, the signals are decoded into analogue signals
• Digital signals can be handled by microprocessors, can carry more info. and can be delivered with no loss of quality
• All signals get weaker as they travel and noise is added in. With digital signals the noise can be corrected
• A re generator restores the digital pulses to their original quality but with analogue, it can be amplified, but the noise isalso amplified
• In optical fibres, digital signals can be sent in the form of pulses of light.
• Optical fibres cannot be tapped and are not effected by electrical interference
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• Radio waves are produced by making electric current oscillate (making charges move to and fro) in a transmitting aerial.
• Radio waves can be sub-divided into groups (UHF, VHF, short wave, long wave and medium).
• To carry info., radio waves must be varied- the variations are called signals
• Radio waves normally travel in straight lines
• Radio stations that use short and medium waves can travel much further
• Long wavelength waves diffract more easily, they can bend around hills and obstacles
• Interference between radio signals from two transmitters can cause the strength of the radio signal to change in strength from place to place
• The radio signals can be reflected off the metal body of aeroplanes
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really helpful i couldn't understand it at all before but now i have seen the light, HALALUYAH

alex:)

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This is a fantastic resource if you need support on the waves topic.

You might feel like you don't understand anything on waves, so have a read of this. It goes through the basics of waves (e.g. what they are) to describing electromagnetic waves.

Useful for all exam boards.

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