Page 195 - Physics_Form_2

Refraction and dispersion of light

Subtraction of colours of light shirt. If the shirt absorbs blue light, it will
The principles of colour addition govern then reflect only red and green light. As
the perceived colour resulting from mixing we see through reflected light, and since,
of different colours of light. However, R+G=Y , the shirt will appear yellow.
understanding colour perceptions cannot . This example illustrates the process
be complete without an understanding of of colour subtraction. In this process,
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the principles of colour subtraction. Some the colour appearance of an object is
materials contain atoms and molecules that determined by identifying colours of light
are capable of selectively absorbing one or that are subtracted from the original set of
more colours of light. Therefore, if a beam colours, as depicted in Figure 5.28.
of white light falls on such a material, some
light colours are absorbed and some are R G B R G
reflected to the observer’s eye. This is the
basis of subtraction of light colours. Figure
5.27 illustrates light colour subtraction. Blue
Yellow filter absorbs blue light, magenta light
filter absorbs green light, and cyan filter absorber
absorbs red light. Figure 5.28: Process of subtraction of light

colours
W=R+G+B W–B W–B–G W–B–G–R
The colour subtraction process illustrated
by Figure 5.28 can alternatively be
depicted using the colour subtraction
equation:

B RG
W  B  (R  G  ) B   . Y
Yellow filter Majenta filter Cyan filter
Suppose that the same piece of cloth is
Figure 5.27: Subtraction of light colours illuminated by a cyan light. Since cyan
consists of blue and green light, and the
Colour subtraction cloth absorbs blue light, then blue light

Consider a piece of cloth that is made of is subtracted. The cloth will then appear
materials capable of absorbing blue light. green. This process is represented by the
Such a cloth will absorb blue light and following equation:
reflect the other colours. What appearance
will such a cloth have if illuminated with C  B  (G  ) B  BG
white light, and how can we account for
its appearance? This observation highlights the fact that
an object’s colour is not part of the object

Suppose that a beam of white light made itself, but depends on the light colours it
of blue, green and red colours falls on a reflects. Hence, the cloth appears yellow

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Physics Form 2 Final.indd 189 25/10/2025 10:28