The Source of Colours Topic #6

What were they thinking?  At one time, people believed that colour was something that was added to light  When white light struck a green leaf, the leaf was adding green to the light  Our good buddy Sir Isaac Newton figured out this wasn’t true in the 17 th century

Such a bright guy  Using prisms, he discovered that bands of colours would emerge from white light  When he passed the colours through a reverse prism, he discovered that the rainbow of colours formed back into white light

Prisms

 When light is refracted into different colours, the resulting pattern is called a spectrum  ROY G BIV (red, orange, yellow, green, blue, indigo, violet) is the pattern for sunlight  This pattern is called the solar spectrum  A rainbow is an example of this solar spectrum

 When light strikes an object, the light may be reflected off the object, absorbed by the object, or transmitted through the object  Example: when light passes through a blue bottle, the glass absorbs all the colours of light except the blue  Only the blue light is transmitted or reflected and so the bottle appears blue

Additive Primary Colours  The three colours red, green and blue are called the additive primary colours  They are called additive because adding all three together in proper amounts will make white light  The 3 secondary colours are yellow, cyan, and magenta

Additive Primary Colours  TVs use additive primary colours  The screen contains groups of three tiny phoshor dots  Each dot glows with a different colour when it receives energy from the electrons inside the picture tube  In each group, one dot glows red, one blue and one green – if all 3 glow you see white light

How We See Colour  The retina of the human eye contains two types of cells that respond to light  Some cells look like tiny cylinders – they are called rods  Rods detect the presence of light  The other cells sort of look like cones and are therefore called cones

 Cones detect colour in our eyes.  There are 3 types of cones (like a TV screen) and each one detects a different colour

Colour Blindness  The cone cells in some people’s eyes are unable to detect certain colours  This condition is called colour blindness