Topic 5: Light and EM Spectrum

• Specular:

  • surface is smooth
  • reflected rays of light are parallel
  • example - mirrors
• Diffuse:

  • surface is rough
  • reflected rays of light are scattered
  • example - paper
• Law of Reflection - the angle of incidence is equal to the angle of reflection and reflected at a right angle on the normal
• When light passes from air (less dense) to glass (more dense), the light slows down and bends towards the normal.

  The wavefronts slow down and the wavelength decreases while frequency remains the same
• White light is a mixture of the 7 rainbow colours of the visible spectrum:

  • When white light hits a green object, the green light of the visible spectrum is reflected into our eyes which makes us see the colour green
  • Other colours are absorbed or transmitted into the object
• The critical angle is the angle of incidence which results in an angle refraction of 90 degrees - the ray travels along the boundary of the material.

  Any angle smaller than the critical angle will refract while ones larger will have total internal refraction (TIR)
• Convex:

  • converging lens
  • refract parallel rays of light inwards to a single point
• Concave:

  • diverging lens
  • refract parallel rays of light outwards to disperse the light
• All lenses:

  • any ray of light parallel through the central axis will pass through the focal point in a straight line
  • any ray of light that passes through the focal point will be parallel to the axis after passing through
  • virtual images are always magnified
  • the more powerful the lens, the shorter the focal length and the closer the focal point is
• Real images are formed when the object is in front of the screen and the image is inverted (e.g. eyes, cameras, projectors)

  Virtual images are formed when the object is in front of the retina but the image is behind the retina (e.g. telescopes, glasses, magnifying glass)
• The Electromagnetic (EM) spectrum is a group of 7 transverse waves with the shared properties:

  • can travel through a vacuum (space)
  • travel at the same speed in a vacuum
• EM Spectrum and Uses:

  • Radio Waves - communication (radio)
  • Microwaves - communication (satellite) and cooking
  • Infrared - cooking (grills) and detecting body heat (thermal cameras)
  • Visible Light - what we see (colours) and photography
  • Ultraviolet - sterilising water and security (detect forgery/security marking)
  • X Rays - security (airports) and health (hospital scans for bone structure)
  • Gamma Rays - treat cancer (radiotherapy) and sterilisation (medical equipment/transporting food)

  Frequency increases, wavelength decreases, energy carried by wave increases and danger increases down the group
• EM Spectrum and Dangers:

  • Radio Waves - safe
  • Microwaves - burns in high concentrations
  • Infrared - burns in high concentrations
  • Visible Light - burns and blindness in high concentrations
  • Ultraviolet - sunburns, skin cancer and retinal damage
  • X Rays - cancer and cell mutations/destruction
  • Gamma Rays - cancer and cell mutations/destruction
• Colours in visible light have different wavelengths and temperatures:

  • red (closest to infrared) - bigger wavelengths are cooler
  • violet (closest to ultraviolet) - smaller wavelengths are hotter

  Absorbing radiation would increase temperatures and emitting radiation would decrease it.
• • Hotter objects contain more energy than cooler objects
  • An object emitting radiation more than absorbing it will decrease in temperature and vice versa
  • An object emitting and absorbing equal amounts of energy will have a constant temperature
• Intensity - the power of the radiation per unit area
• The hotter the object, the shorter the peak wavelength and the higher the area under the graph. As temperature increases:

  • intensity of every emitted wavelength increases
  • intensity of shorter wavelengths increases

  
• In the transmitter ariel, oscillations in an electrical circuit generate radio waves.

  The receiver ariel detects the radio waves which causes it to oscillate as well
• Long waves bend around the curved surface of the Earth.

  Short waves can travel long distances by being reflected along the Earth.

  Very short waves cannot travel very far and they travel in straight lines between the transmitter and receiver (easily obscured)