visible light & colour

• Visible Light

  • Used in cameras, monitor screens and FIBRE OPTIC COMMUNICATIONS, as it can carry large amounts of data over long distances with very little loss of signal.
  • used in communications like in mobile phones.
  • Used in bar code readers
  • Used in safety clothing
  • Cells in the eyes are adapted to detecting visible light, so it's used for imaging.
  • The human eye only detects the visible spectrum (7x10^-7 --> 4x10^-7m wavelength)

  
• The different colours, means different wavelengths.

  The colours are:

  • red
  • orange
  • yellow
  • green
  • blue
  • indigo
  • violet

  A) longest

  B) shortest

  
• visible light


  Used in communications by optical fibres (thin glass or plastic fibres):

  • able to transmit pulses of light over long distances:
  • the light is reflected when it hits the surface.
  • it ends up bouncing back and forth, throughout the entire journey.
  • until it emerges at the other end of the fibre, where it can be interpreted.
  • by encoding information into the light pulses, it can transmit data quickly over long distances.
  • but have to be materials that will reflect the light (than absorbing).
  • To ensure the reflection is specular (than diffuse) so the light isn't scattered at all.

  
• Alternatives of optical fibres are:

  • copper wires
  • electricity

  but optical fibres can:

  • transmit much more info
  • signals are less likely to be distorted (during transmission)

  
• Visible light


  Made up of a whole spectrum of different colours; ROYGBIV.

  White light:

  • a combination of all colours.

  Black:

  • The absence of light.

  
• An object's colour depends on the:

  • Wavelengths of light:
  • hitting the object
  • absorbed by the object
  • reflecting off of the object
  • Properties of the object

  
• Opaque


  Don't transmit any light, so all the wavelengths are either absorbed or reflected.

  • It's the reflected wavelengths that determines the colour.

  An object may reflect a range of wavelengths (combines to give a particular colour)

  Example, an object reflects red and green wavelengths, which combine to make it seem it's reflecting yellow wavelengths.

  
• Transparent / translucent


  Transmit lots of the light that hits them.

  • Transparent:
  • Transmits nearly all the light, only a small fraction gets absorbed / reflected.
  • Translucent:
  • Only transmit some of the light, the proportion of light they transmit determines how well we can see through.

  
• The colour of a translucent object is determined by the wavelengths that's transmitted the most.

  For example:

  • A green glass bottle appears green because:
  • The bottle reflects and/or transmits green light and absorbs other colours of light, so it's not reaching our eye.

  
• Colour filters


  filter out particular colours, so only certain wavelengths can pass through.

  • by only transmitting certain wavelengths of light, while absorbing the rest

  
• Primary colour filter (physics)


  Only allows one of the three primary colours to be transmitted:

  • red
  • green
  • blue

  Example:

  • looking at white paper through a green filter would appear green.
  • As only reflected green waves would make it through.
  • looking at a blue object (reflecting only blue light) through a green filter would appear black.
  • As the filter would block the blue light from passing through and so no light would be reflected into your eye.

  
• Filters that aren't for primary colours, lets through:

  • wavelengths of the same colour as the filter.
  • wavelengths of the primary colour that can be added together to make that colour.

  Example:

  indigo filters transmits:

  • indigo light
  • blue light
  • red light

  but absorbs:

  • orange light
  • yellow light
  • green light
  • violet light

  
• The blue object appears black, because only red is transmitted by the filter and red is absorbed by the blue object, so no light is reflected by the blue object.