Waves

• A wave is a disturbance that transfers energy from one place to another, without transferring matter.
• Wavelength is the distance between the same points on two consecutive waves
• Amplitude is distance from equilibrium line to the maximum displacement (crest or trough)
• Frequency is the number of waves

  that pass a single point per second
• Period is the time taken for a whole

  wave to completely pass a single point
• Period = 1 / frequency
• Transverse waves:

  • e.g. light, or any electromagnetic wave
  • Have peaks and troughs
  • Vibrations are at right angles to the direction of travel
• Longitudinal wave :

  • e.g. sound waves
  • Have compressions and rarefractions
  • Vibrations are in the same direction as the direction of travel
• Remember, for both types of waves, the wave moves and not whatever it pass through
• Sound in air :

  • make a noise at 50 m from a solid wall, and record time for the echo to be heard, then use speed = distance / time
  • Have two microphones connected to a data logger at a large distance apart, and record the time difference between a sound passing on to the other - then use speed = distance time
• Ripples on water surface :

  • use a stroboscope, which has the same frequency as the water waves, then measure distance between the 'fixed' ripples and usevelocity = frequency x wavelength
  • Move a pencil along the paper at the same speed as a wavefront, and measure the time taken to draw this line - then use speed = distance / time
• Electromagnetic waves:

  • these are transverse waves
  • Do not need particles to move
  • In space, all waves have the same velocity (speed of light)
  • they can transfer energy from a source to absorber: microwave source to food, sun emits energy to Earth
• Speed is constant for all electromagnetic waves
• As wavelength decreases, frequency must increase
• As frequency increases, energy of the wave increases
• Eyes :

  • our retina can only detect visible light, a small part of the entire EM spectrum
  • This visible light is still an EM wave like X rays and microwaves
• Refraction : if entering a denser material, it bends towards the normal
• Refraction:

  • if entering a denser material, it bends towards the normal
  • If entering a less dense material, it bends away from normal
• Substances will absorb, transmit, refract or reflect certain EM waves depending on wavelength

  • E.g. Glass, will transmit / refract visible light
  • Absorb UV radiation
  • Reflect IR radiation
  • The material interacts differently for different parts of EM spectrum because the wavelengths (an frequencies) are different
• Some effects in refraction are due to velocity :

  • When light enters a denser medium it slows down
  • Shorter wavelengths slow down more than longer wavelengths
  • E.g. Blue light slows down more than red
• Why does dispersion occur of white light into a prism?

  • The different wavelengths refract a different amount, and therefore spread out creating a rainbow effect
• When refracting, the speed decreases and wavelength decreases too in denser material, the horizontal lines show the "wave-fronts" of the waves.
• Radio waves:

  • Radio waves are produced by oscillations in electrical circuits
  • When radio waves are absorbed they create an alternating current, AC, at the same frequency as the radio waves
• When electrons move to a higher orbits (further from the nucleus)

  • The atom has absorbed EM radiation
• When the electrons fall to a lower orbit (closer to the nucleus )

  • The atom has emitted EM radiation
• If an electron gains enough energy, it can leave the atom to form an ion
• Gamma rays originated from changes in the nucleus of an atom
• • UV light, X-rays and gamma can have hazardous effects on human body tissue.
  • The effects depend on the type of radiation and the size of the does
• Radiation dose: how much exposure leads to harm for a person
• UV - skin ages prematurely, increasing the risk of skin cancer

  • sun cream prevents over-exposure in summer
• X-rays and gamma are ionisation radiation that can cause the mutation of genes - causing cancer

  -Minimal exposure should be ensured
• Use of radio: TV and radio

  • Long wavelength, can travel far without losing quality
• Use of Micro waves: Satellite communication, cooking food

  • can penetrate atmosphere to reach satellites
• Use of IR waves: Cooking food, infrared cameras

  • Transfers thermal energy
• Use of visible waves: Fibre optics

  • Best reflection / scattering in glass (other have too short / long wavelengths)
• Uses of UV waves: Sun tanning, energy efficient lamps

  • Radiates the least heat but more energy
• Uses of X-ray waves: Medical imaging and treatment (an gamma)

  • Very high in energy, and can penetrate material easily