Topic 5 ~ Light and the EM Spectrum

• Ray Diagrams show the direction of light travelling
• Normal


  An (imaginary) dashed line perpendicular to the surface, from which all angles are measured
• Incident Angle


  The angle of the entering ray
• Reflected Angle


  The angle of the exiting ray
• Reflection


  1. Incident angle equals reflection angle

  2. Angles are always measured from the normal
• Refraction


  1. If entering a denser material, light bends towards the normal

  2. If entering a less dense material, light bends away from the normal
• Total Internal Reflection (TIR)


  1. Occurs when light passes from a denser medium into a less dense medium

  2. If angle of incidence equals critical angle, the refracted ray will not exit the medium

  3. For larger angles, light internally reflects back into the denser medium
• Specular Reflection


  Mirror reflection for a smooth surface where all light incident at the same angle exits at the same angle
• Diffuse Reflection


  Light hitting a rough surface is reflected at many angles rather than just one angle
• Colour


  • Each colour is a certain wavelength in visible light
  • All colours together make up white light
• Opaque Material


  Objects appear to have a certain colour as only that certain colour light is reflected, all other colours are absorbed
• Colour Filters


  All other colours are absorbed, and only a certain colour is allowed to pass through, transmitting only a certain wavelength
• Lenses


  • Focal Length is the distance between the lens and the focal point
  • Focal Point is where all horizontal rays meet after passing through the lens
  • Power of the lens is the inverse of the focal length, shorter focal length means greater power
• Concave Lenses


  "Caves" inward, thinner at the centre than at the edges, spreads light outwards, used to spread out light further
• Convex Lenses


  Fatter at the centre, focuses light inwards, used for magnifying glasses, binoculars, and to correct long-sightedness
• Images

  A Real image is produced at the opposite side of the lens to the object
• Horizontal rays


  Focus onto focal point
• Uses of horizontal rays

  • Magnifying glasses
  • Binoculars
  • Correcting long-sightedness
• Real image for a convex lens


  Produced at the opposite side of the lens to the object
• Virtual images


  Appear to come from the same side of the lens to the object if the object lies closer to the lens than the focal point (F)
• All electromagnetic waves transfer energy from source to observer
• Electromagnetic waves contain energy, for example microwaves transfer energy from source to food
• Electromagnetic waves are transverse waves
• All electromagnetic waves travel at the same speed in a vacuum
• EM waves do not need particles to move
• In space, all waves have the same velocity (speed of light)
• EM waves can transfer energy from a source to absorber, e.g., microwave source to food, sun emits energy to Earth
• Our eyes can only detect visible light
• Materials interact with EM waves differently depending on the wavelength, e.g., glass can transmit visible light, reflect/absorb UV and IR
• All bodies emit radiation
• The higher the temperature, the more intense (and more wavelengths) will be emitted
• To remain at a constant temperature, an object must radiate the same average power that it absorbs
• If an object absorbs more power than it emits, the temperature will increase
• If an object absorbs less power than it emits, the temperature will decrease
• The temperature of the earth is maintained by the amount of energy received and emitted from the sun
• Short-wavelength Infra-red radiation from the sun reaches the Earth, some is reflected by the atmosphere, most reaches the surface, the energy is absorbed and re-emitted as longer-length IR radiation, mostly absorbed by the atmosphere (greenhouse gases, CO2 etc.) and keeps the Earth warm
• Higher frequency EM waves have more energy, so exposure can transfer too much energy to cells, causing them to mutate and potentially damage them/causing cancer
• Microwaves can cause internal heating of body cells
• Infra-Red waves can cause skin burns
• UV waves can cause damage to surface cells and eyes, leading to skin cancer