Physics: Waves

Physics: Waves

1. Waves

   • Transverse wave
     • Vibrates/oscillates perpendicular to the direction of energy transfer (up and down)
     • Wavelength: the distance from one peak to the next or one trough to the next
     • Amplitude: the height of a wave peak from the rest position
     • Peak: the maximum height the wave reaches
     • Trough: where the wave reaches its lowest point
     • Examples include electromagnetic waves such as visible light
   • Longitudinal wave
     • Vibrates/oscillates parallel to the direction of energy transfer (side to side)
     • Wavelength: the distance from one compression to the next
     • Compression: where the particles are closest together (high pressure)
     • Rarefaction: where the particles are furthest apart (low pressure)
     • Examples include sound waves such as ultrasounds
   • Frequency: number of waves passing a point per second
   • Mechanical waves: require particles to travel e.g.sound waves
   • Electromagnetic waves: can travel through a vacuum and particles slow down the wavespeed e.g.gamma rays

2. The Wave Equation

   • Wavespeed = frequency x wavelength
   • V = f x λ
   • m/s = Hz x m
   • Hertz is the number of waves per second e.g.1Hz = 1 wave/second
   • f = V / λ
   • λ = V / f
   • Example: A water wave travels 6 metres in 3 seconds and has a frequency of 10Hz. What is its wavelength?
     • 6 metres / 3 seconds = 2 m/s wavespeed 2m/s / 10Hz = 0.2m

3. Reflection

   • Reflection: when a wave bounces off of a surface
   • Shiny materials are very smooth and reflect well
   • Echos are reflections of sound
   • Images from plane (flat) mirrors are: upright (the right way up); laterally inverted (mirror image L=R); and virtual (there is no actual light where the image appears to be)
   • Rule of reflection: angle of incidence = angle of reflection
   • The normal: a line 90 degrees to the surface drawn where the ray hits the surface

4. Refraction

   • When a wave bends or changes direction when it passes from one substance into another
   • We see the most diffraction when the densities are very different or if the wave has to change speed
   • As light ENTERS the glass it refracts TOWARDS the normal (E.T)
   • As light LEAVES the glass it refracts AWAY from the normal (L.A)
   • Different colours in white light refract at different angles which is why we can see different colours

5. Diffraction

   • Where waves pass through a gap and spread out
   • We observe maximum diffraction when the gap size is similar to the wavelength
     • If the gap size is very different to the wavelength, little diffraction would be observed
   • Wavelength of light: 400-700 nanometers
     • Wavelength of sound: 1-20 meters
   • Long wave radio waves diffract to get over mountains and hills

6. Sound waves

   • Pitch
     • Pitch is how high or low a sound is
     • It depends on the frequency of the wave
     • Higher frequency means a higher pitch
     • Lower frequency means a lower pitch
   • Volume
     • Volume is how loud or quiet a sound is
     • You can tell the volume of a sound based on the waves amplitude
     • Louder sounds are caused by larger vibrations with larger amplitudes
     • Quiet sounds are caused by small vibrations with small amplitudes

7. Electromagnetic Spectrum

   • Remember
     • My
       • Instructions
         • Visible
           • Under
             • X ray
               • Glasses
   • Radio waves
     • Microwaves
       • Infrared waves
         • Visible light waves
           • Ultraviolet waves
             • X ray waves
               • Gamma rays
   • Largest wavelength
     • Smallest wavelength
       • Highest frequency
         • Highest energy
     • Lowest frequency
       • Lowest energy
   • Radio waves
     • Uses:wifi, terrestrial television, radio and astronomy
     • Too many radio waves are believed to cause cancer and other illnesses
   • Microwaves
     • Uses: heating food with water or fat in, short-distance communication (phones), weather forecasts and satellite
     • Microwaves could cause thermal damage to cells
   • Infrared waves
     • Uses: Heaters, television remotes, toasters, grills, laser CD players, infrared lamps and thermographic cameras
     • Can cause burns
   • Visible light
     • Uses: visible communication, photography, photosynthesis and astronomy
     • Thermal damage and damage to the retina
   • Ultraviolet
     • Uses: sunbeds, chemistry, forensics, fluorescent light, sealing fillings and marking money to prevent fraud
     • Skin and eye damage if over-exposed
   • X-rays
     • Uses: medical imaging and astronomy
     • Can damage cells, causing mutations which could lead to cancer and cell death
   • Gamma rays
     • Can damage cells, causing mutations which could lead to cancer and cell death
     • Uses: sterilising medical instruments, killing harmful bacteria in food and radiotherapy treatment

8. Fibre optics

   • Uses total internal reflection
   • Visible light and infrared can be sent down these fibres
   • Less interference, more secure
   • No loss of strength over long distances

9. The Doppler Effect

   • As a wave travel towards you it is compresses
     • It's wavelength reduces
   • As a wave travels away from you it is stretched (rarefaction)
     • It's wavelength increases

10. Red Shift

    • Red shift is caused when galaxies are moving away from us
    • The faster the galaxy is moving away, the further the lines shift to the red end of the spectrum
    • Increase in frequency, decrease in wavelength of light from other galaxies