The Source of Colours
Each colour refracts at a different angle, which produces the “rainbow effect”. (red refracts the most; violet refracts the least). Therefore, the different colours are already present in the white light.
What do you think happened if a second prism is added?
White light(sunlight) is the result of mixing of all the different colours. When white light is refracted into different colours, the resulting pattern is a spectrum
ROYGBIV
BLM 3-19
BLM 3-20
Consider THIS!
Objects do not possess colour. There is really no such thing as a “red” tie or “purple” shirt. Objects merely have the ability to reflect certain colours and they absorb the rest.
When white light passes through a blue bottle, the glass absorbs all the colours of light except the blue. Only blue light is reflected back; as a result, the bottle appears blue!
The objects that white light falls upon reflect some colours of light and absorb the rest. For example: if the object is red, all the other colours in the spectrum (violet, indigo,blue, green, yellow, orange) are absorbed, and only red is reflected back to our eyes, so we see the object as red.
If we place the red object under a green light, the object would appear black! Why?
Why do we see the colour white?
-white reflects all the colours
Why do we see the colour black?
-black absorbs all the colours
Additive primary colours (red, green, blue) mix together to create white light (see figure 3.51 on page 233).
The light of 2 additive primary colours produce secondary colours:
1)red + blue = magenta
2)green + red = yellow
3)green + blue = cyan
Draw figure 3.51 into notes.
TV screen us additive primary colours – read page 233.
If you take a magnifying glass close to a colour tv screen, you will observe thousands of tiny dots called pixels. These dots are arranged in groups of three and are made of phosphor so they glow. The dots are very close to each other and have a cross section of about 1/10th of a millimeter. Good tv screens have the highest number of dots per square cm, so that only the blending of colour is seen, not the individual dots. Various shades and intensities are produced when the correct combination of dots start glowing.
The fact that tv screens produced the colour white, supports the theory that additive primary colours produce white light.
BLM 3-21
The retina of the human eye contains 2 types of cells that respond to light:
1) Rods – detect the presence of light
2)Cones – detect the presence of colour.
There are three types of cones – one that responds to red, one to blue and the other to green.
Sometimes cones are defective which results in colour blindness. Colour blindness does not necessarily mean that a person cannot see any colour at all. Some colour-blind people cannot distinguish between red and green. About 10% of males are colour-blind and it is hereditary.
Read “Off The Wall” on page 234.
Colour wheel activity on page 235.
Hand in “What did you find out”
Text questions on page 236 #1-6