Topic 5

• Ray Diagram


  • Arrows show direction of light travelling
  • Normal is an (imaginary) dashed line which is perpendicular to the surface, and from which all angles are measured from
  • Incident Angle is the angle of the entering ray
  • Reflected Angle is the angle of the exiting ray
• Reflection


  1. Incident angle = reflection angle

  2. Angles are always measured from normal
• Refraction


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

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


  1. This occurs when the light is passing from a denser medium into a less dense medium (glass to air)

  2. If the angle of incidence is equal to the critical angle, the refracted ray will pass along the boundary and not exit the medium

  3. The critical angle is a unique angle for each two media (the critical angle for glass-air is different to glass-water)

  4. For larger angles, the light internally reflects (following the above law of reflection) back into the glass
• Summary for glass to air

  • If angle LESS than critical angle, light refracts away from normal
  • If angle EQUAL to critical angle, light passes along boundary
  • If angle MORE than critical angle, light reflects
• Specular Reflection


  Mirror reflection, following law of reflection, for a smooth surface (all light incident at the same angle all exit at the same angle)
• Diffuse Reflection


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


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


  Objects appear to have a certain colour (e.g. 'green'), as out of the incident white light only that certain colour light (green 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 - so only a certain wavelength is transmitted through the filter
• Lenses


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


  • Caves inward
  • Thinner at centre than at edges
  • Spreads light outwards
  • Light appears to have come from the focal point
  • It is used to spread out light further
• Convex Lenses


  • Fatter at centre
  • Focuses light inwards
  • Horizontal rays focus onto focal point
  • They are used for magnifying glasses, binoculars and to correct long-sightedness, as it focuses the rays closer
• Images


  • A Real image is an image 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
• Electromagnetic Waves


  • All electromagnetic waves transfer energy from source to observer
  • They are transverse waves
  • They all 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)
  • They can transfer energy from a source to absorber
  • Our eyes can only detect visible light
  • Materials interact with EM waves differently depending on the wavelength
• As speed is constant for all EM waves in a vacuum

  • As wavelength decreases, frequency must increase
  • As frequency increases, energy of the wave increases
• All Bodies emit radiation
• Temperature of an object

  • It must radiate the same average power that it absorbs to remain at a constant temperature
  • If It absorbs more power than it emits –the temperature will increase
  • If it absorbs less power than it emits – the temperature will decrease
  • Temperature of the earth – this is maintained by the amount of energy received and emitted from the sun
• Uses of the EM spectrum

  • Radio - Communications, satellite transmission
  • Microwave - Cooking, communication
  • IR - Cooking, thermal imaging, short range communication, optical fibres
  • Visible - Vision, photography, illumination
  • UV - Security marking, fluorescent lamps, disinfecting water
  • X-ray - Observing internal structure of objects, airport/medical scanners
  • Gamma - Sterilising food/medical equipment, treating cancer
• radio waves?


  Communications, satellite transmission
• Microwaves?


  Cooking and communication
• Infrared?


  Thermal imaging , optical fibres
• Visible ?


  vision , photograpghy
• Ultraviolet?


  Tanning , flouresent lamps
• x-ray?


  medical scanning
• gamma?


  sterilising food , treating cancer
• shorter focal length?


  greater the power