Topic 5 - Light and the EM spectrum

• Ray Diagrams


  • Arrows show direction of light travelling
  • The 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

  4. For larger angles, the light internally reflects back into the glass
• 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

  1. Each colour is just a certain wavelength in visible light

  2. All the colours together make up white light
• Opaque Material


  Objects appear to have a certain colour as out of the incident white light 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 - so only a certain wavelength is transmitted through the filter
• Lenses

  1. Focal Length is the distance between the lens and the focal point

  2. Focal Point is the point where all horizontal rays meet after passing through the lens

  3. Power of the lens is the inverse of the focal length

  4. Shorter focal length, greater power

  5. Thicker lens means shorter focal length, so greater power
• Concave Lenses


  1. “Caves” inward

  2. Thinner at centre than at edges

  3. Spreads light outwards

  4. Light appears to have come from the focal point

  5. It is used to spread out light further
• Convex Lenses


  1. Fatter at centre

  2. Focuses light inwards

  3. Horizontal rays focus onto focal point

  4. They are used for magnifying glasses, binoculars and to correct long-sightedness
• Images

  A Real image is an image produced at the opposite side of the lens to the object
• Horizontal rays focus onto focal point


  Used for magnifying glasses, binoculars, and to correct long-sightedness, as it focuses the rays closer
• Images

  • Real image is 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 which transfer energy from source to food
• Electromagnetic waves are transverse waves
• All electromagnetic waves travel at the same speed in a vacuum
• Electromagnetic waves do not need particles to move
• In space, all waves have the same velocity (speed of light)
• Electromagnetic waves can transfer energy from a source to absorber, for example, microwave source to food, sun emits energy to Earth
• Our eyes can only detect visible light
• Materials interact with electromagnetic waves differently depending on the wavelength, for example, glass can transmit visible light, reflect/absorb UV and IR
• As speed is constant for all electromagnetic waves in a vacuum

  As wavelength decreases, frequency must increase
• As frequency increases


  Energy of the wave increases
• All bodies emit radiation
• The higher the temperature, the more intense (and more wavelengths) will be emitted
• Temperature 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 is maintained by the amount of energy received and emitted from the sun
• Short-wavelength infrared 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 electromagnetic waves have more energy, so exposure can transfer too much energy to cells, causing them to mutate and potentially damage them/causing cancer
• 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
• Change in Atoms and Nuclei can generate radiations over a wide frequency range and be caused by absorption of a range of radiation