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7.5 Addition and Subtraction of Coloured

LightsA. Addition of Coloured LightsColours may classified into two groups:

a) Primary colours RED,GREEN and BLUE

- cannot be obtained by mixing other colours.RED + GREEN + BLUE

b) Secondary colours- CYAN,MAGENTA and

- obtained by mixing two primary colours.RED + GREEN

RED + BLUE MAGENTA

GREEN + BLUE CYAN

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c) Complementary Colours

- a pair of two colours, a primary and a secondary colour which

combine to form white light.

+ BLUE

MAGENTA + GREEN

CYAN + RED

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B. Subtraction of Coloured Lights by Coloured Filters1. Filter is a transparent piece of glass, plastic or gelatin sheets which

allows some light through and prevent others.2. The action of filtering (preventing other colours) is called

subtraction of coloured lights.

3. 2 types of filter:

a) primary filter (red, blue, green)

-only allows its own colour to pass through.

-absorb all other colours.

- e.g Red filter only allows red light to pass through it.

b) secondary filter (magenta,cyan,yellow)

- only allows its own colour and its component to passthrough it.

- e.g Yellow filter only allows red, green and yellow to passthrough it.

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Primary Filters

White light

White light

White light

red

green

blue

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White light

Secondary Filter

White light

White light

red

green

yellow

blue

green

cyan

red

blue

magenta

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White light

White light

White light

Red is common to both filters.

Green is common to both filters.

BLACK

No common colour.

All colours are absorbed.

Yellow filter allows red,

green and yellow

colour to pass through

it.

Cyan filter allows

blue, green and

cyan colour to

pass through it.

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7.6 Appearance of Coloured Objects

A. Coloured Objects1. Colour of an object depends on the

a) the colour of light that shines on the object

b) the colour of light absorbed by the the object

c) the colour of light reflected to the eyes of observer

2. Primary coloured objects absorb all colours except their own

colours which will be reflected to the observers eyes.

white red red red

blueObject looks black as

the blue light is

absorbed.

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3. Secondary coloured objects absorb all colours except their own

colour and their primary component colours.

white yellow yellow yellow

green green red red

blue

Object looks black as blue

light is absorbed.

Yellow = red + green

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4. White coloured objects looks white because it reflects all the

colours in white light.

5. Black coloured objects looks blacks because it absorbs all colours in

white light.

white white red red

white greenAll the colours

in white light

are absorbed.

Green light is

absorbed.

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B. Rods and Cones in The Retina

1. Retina of the eye contains two types of sensory cells called rods and

cones. When they are stimulated by light from an object, they send

impulses along nerve fibres through the optic nerve to the brain.

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2. Rods;

a) are more suitable for seeing at night as they are more

sensitive to dim light.b) do not see colour, see the objects in black and white.

c) identify the different forms and shapes of objects in dim

lights.

3. Cones;

a) see colour of objects in bright light only, suitable for seeing

in day light.

b) have 3 types, each is sensitive to one primary colour.Together, they tell us the colour of an object.

c) yellow spot (most sensitive part of the retina) composed of

cones only.

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7.7 Pigments1. Pigments are coloured substances, obtained from plants and most

are artificially from chemicals.

2. They are used in paints and dyes.

3. Examples of pigments; chlorophyll (green), charcoal(black) and

turmeric (yellow).

4. Most pigments are not pure and can reflect more than onecoloured light.

5. Primary colour pigments are red, blue and yellow.

6. Secondary colour pigment will be produced when two primary

colour pigments are mixed, green, violet and orange.

7. White cannot be obtained by mixing the three primary colours.

8. Black is obtained when these three primary pigments are mixed

in correct proportions.

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8. The result of mixing of light is not the same as mixing pigments ascoloured pigments are not pure.

9. The formation of colours by mixing pigments is a colour subtractionprocess.

Example 1

- Yellow pigment reflects yellow, green and red.

- Cyan pigment reflects cyan, green and blue.

- Both pigments reflects green colour.

- So the result of mixing the two pigments produces green colour.

- Green colour is the common colour.

White

light

White

light Whitelight

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Example 2

- Red pigment reflects red and yellow.

- The green pigment reflects green, yellow and blue.

- Yellow is reflected by both pigments.

- So the mixture of red and green pigments produces yellow colour.

- Yellow is the common colour.

White

light

White

light White

light

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DifferencesDifferences Mixing of Pigments Addition ofColours of

Light

Based on the principle

of

Subtraction of Colours Addition of Colours

Primary Colours Red, blue and yellow Red,blue and green

Secondary colours Green,violet and orange Yellow, cyan and

magenta

Each has its own set of primary and secondary colours

These primary and secondary colours can be mixed or

added to obtain other colour.

Similarities