waves

• Waves


  Transfer energy without transferring matter; particles oscillate about a fixed point
• Wave properties

  • Amplitude
  • Wavelength
  • Frequency
  • Speed
• Amplitude


  The distance from the equilibrium position to the maximum displacement
• Wavelength


  The distance between a point on one wave and the same point on the next wave
• Frequency


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


  The distance travelled by a wave each second
• Relationship between speed, frequency and wavelength

  Speed = Frequency x Wavelength
• Types of waves

  • Transverse
  • Longitudinal
• Transverse waves

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

  • Consists of compressions and rarefactions
  • Vibrations are in the same direction as the direction of travel
• Wavefront


  A surface containing points affected in the same way by a wave at a given time such as crests or troughs
• Reflection

  1. Waves reflect off smooth, plane surfaces

  2. Angle of incidence = angle of reflection

  3. Rough surfaces scatter the light in all directions

  4. Frequency, wavelength, and speed are all unchanged
• Refraction

  1. The speed of a wave changes when it enters a new medium

  2. If the wave enters a more optically dense medium, its speed decreases and it bends towards the normal

  3. If the wave enters a less optically dense medium, its speed increases and it bends away from the normal

  4. Frequency stays the same but the wavelength changes
• Diffraction

  1. Waves spread out when they go around the sides of an obstacle or through a gap

  2. The narrower the gap or the greater the wavelength, the more the diffraction

  3. Frequency, wavelength, and speed are all unchanged
• Reflection of light

  • Upright
  • Same distance from the mirror as the object
  • Same size
  • Virtual
• Refraction of light

  1. When light enters a more optically dense medium, the angle of incidence is greater than the angle of refraction

  2. When light enters a less optically dense medium, the angle of incidence is less than the angle of refraction
• Refractive index

  The ratio between the speed of light in a vacuum and the speed of light in the medium
• Snell's law

  n = sin i / sin r, where i is the angle of incidence and r is the angle of refraction
• Total internal reflection


  1. At the critical angle, the light will travel along the boundary between the two media

  2. Total internal reflection occurs when the angle of incidence is greater than the critical angle and the light reflects back into the medium

  3. For total internal reflection, the light must be travelling from a more optically dense medium into a less optically dense medium
• Critical angle


  The angle of incidence at which total internal reflection occurs
• Optical fibres


  • Long thin rod of glass surrounded by cladding
  • Uses total internal reflection to transfer information by light, even when bent
  • Extensive use in medicine and communications
• Converging lens


  • Brings light rays together at a point called the principal focus
  • The focal length is the distance between the centre of the lens and the principal focus
• Real image


  Image where light actually converges to a position and can be projected onto a screen
• Virtual image


  Image where light only appears to have converged and they cannot be projected onto a screen
• Converging lenses are used in magnifying glasses and binoculars (to enlarge the image)
• Dispersion

  1. White light splits up into its constituent colours when passed through a glass prism

  2. The different colours travel at different speeds in the glass, so they refract by different amounts

  3. The greater the wavelength, the slower the speed in glass and the greater the refractive index
• Monochromatic light


  Light of a single frequency
• Properties of electromagnetic waves

  • Transverse
  • Do not need a medium
  • All travel at the same high speed of 3.0 x 10^8 m/s in a vacuum and approximately the same speed in air
• Groups of the electromagnetic spectrum in order of wavelength

  • Radio waves
  • Microwaves
  • Infrared
  • Visible light
  • Ultraviolet
  • X-rays
  • Gamma rays
• As wavelength decreases


  Frequency must increase
• The higher the frequency of an EM wave


  The greater its energy
• electromagnetic waves

  • Radio waves - radio, TV transmissions, astronomy, radio frequency identification (RFID)
  • Microwaves - satellite television, mobile phones, microwave ovens
  • Infrared - electric grills, short-range communications like remote controllers for tv, thermal imaging, intruder alarms, optical fibres
  • Visible light - optical fibres, vision, photography, illumination
  • Ultraviolet - security marking, detecting fake bank notes, sterilising water
  • X-rays - medical imaging and security scanners
  • Gamma radiation - sterilising food and medical equipment, detection of cancer and treatment
• Hazards of electromagnetic waves

  • Ultraviolet light increases skin cancer risk
  • X-rays and gamma rays can cause mutations leading to cancer
  • Microwaves can cause internal heating of body tissues
  • Infrared radiation can cause skin burns
• Sound waves


  Longitudinal waves created by vibrating sources, requiring a medium to transmit
• Amplitude of sound wave

  Determines the loudness
• Frequency of sound wave

  Determines the pitch
• Measuring speed of sound

  Make a noise at a known, large distance from a solid wall and record the time for the echo to be heard, then use speed = distance/time
• Speed of sound in air is 343 m/s, in water is 1493 m/s, and in steel is 5130 m/s
• Audible frequency range for humans

  20 Hz to 20000 Hz
• Ultrasound


  Sound with a frequency greater than 20000 Hz