Everything Science Grade 12

CHAPTER 10. COLOUR 10.4

Colour of opaque objects ESCEW

Objects which you cannot see through (i.e. they are not transparent) are called opaque.
Examples of some opaque objects are metals,wood and bricks. Thecolour of an
opaque object is determined by the colours (therefore frequencies) of light which it
reflects. For example, when white light strikes a blueopaque object such asa ruler,
the ruler will absorb allfrequencies of light except blue, which will be reflected. The
reflected blue light is the light which makes it into our eyes and therefore the object
will appear blue.

FACT

Colour printers only use

4 colours of ink: cyan,

magenta, yellow and

black. All the other

colours can be mixed

from these!

Opaque objects whichappear white do not absorb any light. They reflect all the fre-
quencies. Black opaqueobjects absorb all frequencies of light. They donot reflect at
all and therefore appearto have no colour.

Example 5: Colour of Opaque Objects

QUESTION

If we shine white lighton a sheet of paper that can only reflect greenlight,

what is the colour of thepaper?

SOLUTION

Since the colour of anobject is determined bythat frequency of light that is

reflected, the sheet of paper will appear green, as thisis the only frequency

that is reflected. All theother frequencies are absorbed by the paper.

Example 6: Colour of an opaque object II

QUESTION

The cover of a book appears to have a magentacolour. What colours oflight

does it reflect and whatcolours does it absorb?