1. Colour by Subtraction
SNC2P – Optics

2. 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.

3. 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

4. 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

5. 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

6. 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.

7. 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.

8. 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.

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

10. Practice Complete the following subtraction colour theory equations.
R + B =
C + M + Y =
M =
Y + M = M B
R + B R

11. Homework State the colour seen in the following cases:
Cyan transparent filter
Blue transparent filter
Magenta transparent filter
Complete the following:
B = -
M =
Black =
R = M -