Waves - Topic 6

• Wavelength


  Distance between the same points on two consecutive waves
• Amplitude


  Distance from equilibrium line to the maximum displacement (crest or trough)
• Frequency


  The number of waves that pass a single point per second
• Period


  The time taken for a whole wave to completely pass a single point
• Calculating wave velocity


  velocity = frequency × wavelength = 𝑣𝑣 = 𝑓𝑓𝑓𝑓
• Calculating period


  period = 1/frequency = 𝑇𝑇 = 1/𝑓𝑓
• Increase frequency


  Velocity increases
• Wavelength increases


  Velocity increases
• Smaller period


  Higher frequency, greater velocity
• Transverse waves


  • Have peaks and troughs, Vibrations are at right angles to the direction of travel
• Longitudinal waves


  • Have compressions and rarefactions, Vibrations are in the same direction as the direction of travel
• For both transverse and longitudinal waves, the wave moves and not whatever it passes through
• Measuring sound velocity in air

  1. Make a noise at ~50m from a solid wall, and record time for the echo to be heard, then use speed = distance/time

  2. Have two microphones connected to a datalogger at a large distance apart, and record the time difference between a sound passing one to the other – then use speed = distance/time
• Measuring ripple velocity on water surface

  1. Use a stroboscope, which has the same frequency as the water waves, then measure distance between the 'fixed' ripples and use 𝑣𝑣 = 𝑓𝑓𝑓𝑓

  2. 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
• Reflection


  • Waves will reflect off a flat surface, The smoother the surface, the stronger the reflected wave is, Rough surfaces scatter the light in all directions, The angle of incidence = angle of reflection, Light will reflect if the object is opaque and is not absorbed by the material
• Transmission


  • Waves will pass through a transparent material, The more transparent, the more light will pass through the material
• Absorption


  • If the frequency of light matches the energy levels of the electrons, The light will be absorbed by the electrons and not reemitted, They will be absorbed, and then reemitted over time as heat
• Sound wave transmission in the ear

  Sound waves hit the eardrum, Compression forces the eardrum inward, Rarefaction forces the eardrum outward, due to pressure, The eardrum vibrates at the same frequency as the sound wave, The small bones connected to this also vibrate at the same frequency (stirrup bone), Vibrations of the bones transmitted to the fluid in the inner ear, Compression waves are thus transferred to the fluid (in the cochlea), The small hairs that line the cochlea move too, Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies, The hairs each come from a nerve cell, when a certain frequency is received, the hair attuned to that specific frequency moves a lot, releasing an electrical impulse to the brain, which interprets this to a sound
• Humans cannot hear below 20Hz or above 20kHz
• In the cochlea, the hairs attuned to the higher frequencies die or get damaged due to constant loud noise, aging, smoking, chemotherapy, diabetes
• We have evolved to hear this range of frequencies as it gives us the greatest survival advantage
• Ultrasound


  • When ultrasound reaches a boundary between two media, they are partially reflected back, The remainder of the waves continue and pass through, A receiver next to the emitter can record the reflected waves, The speed of the waves are constant, so measuring the time between emission and detection can show distance from the source they are
• Infrasound


  • It is a sound wave with a frequency lower than 20Hz, P waves are longitudinal and can pass through solids and liquids, S waves are transverse and only pass through solids
• Sonar


  • Pulse of ultrasound is sent below a ship, and the time taken for it to reflect and reach the ship can be used to calculate the depth
• Electromagnetic waves


  • They 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
• As wavelength decreases


  Frequency must increase
• As frequency increases


  Energy of the wave increases
• Our retina can only detect visible light, a small part of the entire EM spectrum
• 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, When light enters a denser medium, it slows down, Shorter wavelengths slow down more than longer wavelengths
• Dispersion occurs of white light into a prism because the different wavelengths refract a different amount, and therefore spread out creating a rainbow effect
• Radio waves


  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
• Atoms and EM radiation


  When electrons change orbit (move closer or further from the nucleus), When electrons move to a higher orbit (further from the nucleus) the atom has absorbed EM radiation, When the electrons falls to a lower orbit (closer to the nucleus) the atoms has emitted EM radiation, If an electron gains enough energy, it can leave the atom to form an ion, Gamma rays originate from changes in the nucleus of an atom
• Hazards of EM radiation

  • 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 dose, UV – skin ages prematurely, increasing risk of skin cancer, X-ray and gamma are ionisation radiation that can cause the mutation of genes – causing cancer
• Uses of EM waves

  • Radio - TV and radio, Micro - Satellite communication, cooking food, IR - Cooking food, infrared cameras, Visible - Fibre optics, UV - Sun tanning, energy efficient lamps, X-ray - Medical imaging and treatment (and gamma)
• Convex lenses


  • They focus light inwards, Horizontal rays focus onto focal point, Used for magnifying glasses, binoculars, Used to correct long-sightedness, as it focuses the rays closer
• Concave lenses


  • They "cave" inward, They are thinner at centre than at edges, Spreads light outwards, Light appears to have come from the focal point, Used to spread out light further, e.g. to correct short-sightedness
• Magnification


  magnification = image height/object height
• Specular reflection


  • Smooth surface gives a single reflection
• Diffuse reflection


  • Reflection off a rough surface causes scattering
• Colour filters


  They work by absorbing every other colour and only letting certain wavelength (i.e. a certain colour) through