14.

7 The visible
1.1 world

Look around and you will see many colours

around you.
Why is the world
colourful to us?
A The visible spectrum
The light that the human eye can see is
called visible light (可見光).

The Sun is an important source of visible

light to us. Sunlight appears white.
By using a prism or diffraction grating,
white light can be separated into different
colour
 14.16

Discussion
What can you conclude from the experiment?
White light _______________ (can / cannot)
be separated into different colours of light.
The different colours of light ______________
(can / cannot) be combined to form white light
again.
A The visible spectrum
• White light is a mixture of different colours of
light.

• Different colours of light bend by different

amounts when they enter and exit the prism.
 bends the least
 bends the most
A The visible spectrum
White light spreads into a band of colours by a prism

 the band of colours:

visible spectrum (可見光譜) or colour spectrum
A The visible spectrum
Different colours have different
wavelengths (波長).
colour wavelength (nm)
6
, 65 X 10 m
red 665
minimum distance that
a wave repeats itself
orange 600
yellow 580
green 550

blue 470

indigo 430
violet 400
A The visible spectrum

• White light is a mixture of different

colours.
• Different colours of light have different
wavelengths.
B The three primary colours of light
light of a band of colours

combine

white Light of red, green and

light blue can do the same.
 14.17
Video

Mixing red, green and blue light

(Teacher demonstration)
Aim
To produce different colours by
mixing red, green and blue light
 14.17
1 Your teacher will insert a colour filter to each ray
box to project red, green and blue light onto a
screen as shown.

ray boxes with colour filters

 14.17
2 Darken the laboratory by switching off the lights
and drawing the curtains.
 14.17
3 Mix the colours of light in the table given and
write down the names of the colours you
observed.
B The three primary colours of light
Primary colours of light can be combined to
produce different colours of light.

red + green = yellow

B The three primary colours of light

red + blue = magenta (洋紅)

B The three primary colours of light

green + blue = cyan (藍綠)

B The three primary colours of light

red + green + blue = white

 14.17 red

magenta yellow

Results white

blue cyan green

Colours of light mixed Resulting colour of light

 red +  green
yellow
 red +  blue
magenta
 green +  blue
Cyan
 red +  green +  blue white
B The three primary colours of light

• Red, green and blue are the

_______________ colours of light.
B The three primary colours of light

Light of red, green and blue

primary colours (原色) of light

RGB
All colours can be produced by mixing the CMYK
primary colours of light in different proportions.
g
xG
absorb *
white A Fred
light & ↑·
reen
green
and
ink
lightIf absorb
green
and
blue
light
⑤ SY -
R
xG

blue
red object blue ink ink
B The three primary colours of light

• All colours of light can be produced by mixing

these three colours of light in different proportions.

red

magenta yellow

white

blue cyan green

Why these 3 colours?

C What colour does an object
appear?
The colour that an object appears to us
depends on what colour(s) of light from
the object enters our eyes.
C What colour does an object
appear?
1 In white light
• When white light shines on an object, the
object absorbs some colours of light and
reflects others.
• The colour that the object appears to us is a
combination of the colours of light that the
object reflects.
1 In white light absorb all colours in
white light the white light

appears appears appears

red green blue

appears appears
black white

appears appears appears

magenta yellow cyan

reflect all colours in

the white light
C What colour does an object
appear?
2 In colour light

14.18 Video

Objects in different colours of light

Aim
To find out the colours of objects in
different colours of light
 14.18
1 Observe the colours of different objects
under normal lighting (white light) in the
laboratory. Record their colours in the table
given.
 14.18
2 Darken the room by switching off the lights
and drawing the curtains.
Use a computer-controlled projector to
project red light on to the differently
coloured objects.

red
light
 14.18
3 Observe the colours of the objects and
record them in the table.

4 Repeat step 3, projecting green light and

then blue light onto the objects.

green blue
light light
 allow red light to
pass through
14.18 absorb green and blue
light

=>Filed > A
-

Results filter green

yellow filter - blue filter filter

Colour of the object seen in

Colour
of object white light red light green light blue light

white white red green blue

red red red black black
green green black green black
blue blue black black blue
yellow yellow red green black
C What colour does an object
appear?
When a colour light falls on an object:

• if the object reflects the colour light

 the object appears to be the same colour as the
reflected light

• if the object does not reflect any of the colour light

 it appears black
C What colour does an object
appear?
Simulation

original colour as seen under white light

C What colour does an object
appear?

original colour as seen colour as seen under

under white light red light
C What colour does an object
appear?

original colour as seen colour as seen under

under white light green light
C What colour does an object
appear?

original colour as seen colour as seen under

under white light blue light
C What colour does an object
appear?
The colour of an object is determined by
the colour(s) of light _______________
(absorbed / reflected) by the object.

https://www.youtube.com/watch?v=WotHaxyUtqM
Additive (light) Subtractive (ink)
 14.7
1

white light

visible
prism spectrum

White light is a mixture

of different colours
 14.7
1 White light spreads into the
visible spectrum when
passes through a prism

white light

visible
prism spectrum
 14.7
1

white light

visible
prism spectrum

Different colours of light

have different wavelengths
 14.7
2 Red, green and blue are the primary
colours of light.
All colours can be produced by mixing these
three colours of light in different proportions.

3 The colour of an object is determined by the

colour(s) of light reflected by the object.
 14.7 Level 1

1 Visible light has only three colours.

T F
 14.7 Level 1

2 The three primary colours of light are

cyan, magenta and yellow.

T F
 14.7 Level 1

3 A red apple appears red because it

absorbs red light.

T F
 14.7 Level 1

4 Write down a source of white light.

(1 mark)

Sunlight. (1)

(Or other reasonable answers)

 14.7 Level 2

5 The photo below shows that some green

seedlings are put under an LED grow lamp.
The lamp emits only limited colours of light
that are essential for the plant to grow.
 14.7 Level 2

5 Which primary colour of light does the

LED grow lamp NOT emit? (1 mark)
Green. (1)
 14.7 Level 2

6 An object is shone with red, green and

blue light in turn as shown below.

red light green light blue light

appears appears appears

black green blue
 14.7 Level 2

6 What is the colour of the object? (1 mark)

Cyan. (1)

red light green light blue light

appears appears appears

black green blue