Colour by SubtractionSNC2P – Optics

Transmission, Reflection, and Absorption• A transparent medium is a material that allows

light to pass through it easily. Examples: glass, air, and plastic wrap

• A translucent medium is a material that is partially see through. Examples: frosted glass and wax paper

• An opaque medium is a material that does not allow any light to pass through. Examples: textbook and you• Opaque materials gather the light that strikes

them in a process called absorption.

Colours of Objects• Since most objects are non-luminous, the majority of the

colours we see is light reflecting from objects to our eye

• Pigments are chemicals that absorb some colours of light and reflects other colours. They are naturally occurring and man made. Examples: ink, dyes, and paints

• This means that when you see blue, what you are seeing is all colours being absorbed except blue which is being reflected

Subtractive Colour Theory• The subtractive colour theory states that the apparent colour

of a non-luminous object depends on which colours of light it absorbs and which colours of light it reflects or transmits

Primary and Secondary Colours of Pigments• The primary colours of pigment are

cyan, magenta, and yellow.• The secondary pigments are red, green,

and blue and are formed by mixing of the primary colours of pigments. Example: M + C = B

Explaining Black• When all the primary colours of pigment are mixed together,

the mixture absorbs all three primary colours of light.

• Since no light is reflected, the mixture appears black.

Opaque Objects in White and Coloured Light• Opaque objects contain pigments that absorb some colours

and reflect others. What we see is the colours that are not absorbed by the process. When white light (RGB) shines on a yellow rose, only the red light and green light are reflected (the blue is absorbed). We see the light as yellow.

Opaque Objects in White and Coloured Light• In magenta light (RB), the blue light is again absorbed and red

light is reflected. The rose appears red.

Filters• A transparent material that contains a pigment• Some colours of light (RGB) are absorbed and some are

transmitted

Practice

1. Complete the following subtraction colour theory equations.a) R + B = b) C + M + Y = c) M = d) Y + M =

Homework

1. State the colour seen in the following cases:a) Cyan transparent filterb) Blue transparent filterc) Magenta transparent filter

2. Complete the following:a) B = - b) M = -c) Black = + +d) R = M -