waves

• Transverse wave


  A wave where the particles vibrate at right angles to the direction of travel of the wave front
• Longitudinal wave


  A wave where the particles vibrate parallel to the direction of travel of the wave front
• Sound wave is longitudinal
• Light wave is transverse
• Demonstrate longitudinal or transverse wave in a slinky spring

  1. Lie slinky on stretched out on desk

  2. Wobble one end to produce a wave

  3. To produce a transverse wave, wobble at right angles to the slinky

  4. To produce a longitudinal wave, wobble along the line of the slinky
• Water wave is transverse
• Transverse waves


  Have peaks and troughs
• Longitudinal waves


  Have compressions and rarefactions
• Frequency of a wave

  The number of waves that pass a point each second
• When sound waves move through air, it is the wave and not the air itself that travels. Evidence: e.g. when you hit a drum, there is not a vacuum creates as all the air rushes away
• Unit of frequency


  Hertz (Hz)
• Wavelength


  The distance between two peaks or two troughs on a wave
• Unit of wavelength


  Metre
• Amplitude


  The maximum displacement (movement) of a point on a wave
• Waves


  Transfer energy without transferring matter
• Period of a wave

  The time taken for a wave to pass a point
• Frequency and period


  f = 1 ÷ T or T = 1 ÷ f
• Wavefront


  A line of identical points on a set of adjacent waves, all moving together
• Measure the speed of sound waves in air

  1. Stand a fixed distance from a high wall

  2. Measure distance to wall - trundle wheel

  3. Make a short loud sound (fire a gun!)

  4. Time how long it takes to hear the echo

  5. Repeat many times and average

  6. Speed = 2 × distance ÷ average time
• Measure the speed of ripples on a water surface

  1. Fill a ripple tank with water

  2. Make waves with a wooden bar and a vibrator

  3. Observe the waves projected below the tank

  4. Mark out a fixed distance with a ruler

  5. Use a stop clock to measure the time taken

  6. Repeat many times and average

  7. Speed = distance ÷ average time
• When sound travels from air into a solid, it speeds up. The frequency stays the same (this sets the pitch, which does not change). Using v = f λ, if v decreases and f is fixed, then λ must decrease as well
• When sound travels from a solid into air, it slows down. The frequency stays the same (this sets the pitch, which does not change). Using v = f λ, if v increases and f is fixed, then λ must increase as well
• Measure the frequency of waves in a ripple tank

  1. Fill a ripple tank with water

  2. Make waves with a wooden bar and a vibrator

  3. Observe the waves projected below the tank

  4. Count the number of waves passing a point in 10 seconds

  5. Divide by 10 to find the number of waves passing a point in 1 second. This is the frequency
• Measure the frequency of waves in a solid (e.g. a string)

  1. Put a string under tension

  2. Make waves with a vibrator

  3. Observe the waves with a strobe light

  4. Adjust the strobe light until the moving string appears stationary

  5. Read the frequency from the strobe

  6. Repeat and average
• Measure the wavelength of waves in a ripple tank

  1. Fill a ripple tank with water

  2. Make waves with a wooden bar and a vibrator

  3. Observe the waves projected below the tank

  4. Use a strobe to "freeze" the image of the waves

  5. Measure the distance across 10 waves

  6. Divide by 10 to find the distance across 1 wave. This is the wavelength
• Measure the wavelength of waves in a solid (e.g. a string)

  1. Put a string under tension

  2. Make waves with a vibrator

  3. Observe the waves on the string

  4. Measure the length of the string

  5. Divide by the number of half wavelengths, and double your answer. This is the wavelength

  6. Repeat and average
• Measure the speed of waves in a ripple tank

  1. Fill a ripple tank with water

  2. Make waves with a wooden bar and a vibrator

  3. Observe the waves projected below the tank

  4. Mark out a fixed distance with a ruler

  5. Use a stop clock to measure the time taken

  6. Repeat many times and average

  7. Speed = distance ÷ average time
• Measure the speed of waves in a solid (e.g. a string)

  1. Put a string under tension

  2. Measure the length of the string with a ruler

  3. Make a wave by plucking the string

  4. Use a stop clock to measure the time taken from the wave to travel down the string and back

  5. Repeat and average

  6. Speed = 2 × distance ÷ average time
• Reflection of waves


  Waves can be reflected at the boundary between two different materials
• Absorption and transmission of waves

  Waves can be absorbed or transmitted at the boundary between two different materials
• Law of reflection


  Angle of incidence = Angle of reflection
• Experiment to show refraction in a glass block

  1. Use a ray box or laser as light source

  2. Shine in to glass block

  3. Mark position of rays

  4. Use protractor to measure angle of incidence and angle of refraction
• Differences in velocity, absorption and reflection between different types of wave can be used for the exploration of structures which are hidden from direct observation
• Use ultrasound to detect cracks or faults in solid metal castings


  1. Direct ultrasound pulse through material

  2. If there is a single reflected pulse then there are no faults (reflection is from end of casting)

  3. Other reflected pulses are due to faults

  4. Measure time for each reflected pulse to return

  5. Calculate distance travelled by pulse: distance = speed × time

  6. Distance to each fault is half this distance (pulse travelled to fault and back)
• Sounds


  Produced when objects vibrate
• Range of human hearing

  20 Hz - 20,000 Hz (although most adults cannot actually hear as high as 20,000 Hz)
• Sounds too high for humans to hear

  Ultrasound
• Ultrasound


  Sound waves with a frequency above the upper limit of human hearing
• Sound cannot travel through a vacuum
• Frequency of a sound wave


  Corresponds to the note/pitch