4.3 Properties of Visible Light

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Refraction of Light
• If the light wave is travelling at an
angel and the speed of light travels
in a different angle, the wave will
be bent, or refracted.
Refraction of Light
• Prisms and water droplets refract light
in the same way.
The Visible Light Spectrum
• What would you expect to see if white
light was sent through a prism?
• Lights of different colours (red, green,
and blue) shine through!
• You have just split white light into the
visible light spectrum.
Watch:
https://www.youtube.com/watch?v=x8FZ7
Tu98Yo
• Colour is created from wavelengths of
light.
• Red has the longest wavelengths, 630650 nanometers.
• Blue and purple have the shortest
wavelengths, 380-480 nanometers.
• We have two kinds of light receptors
in the back of our eye that help us
see colour.
• Cones – help us see colour
• 6-7 million per eye
• Rods – help us see in dim light
(black or white) and to use our
peripherals (looking from side to
side)
• Over 120 million per eye.
The Visible Light Spectrum
• Light is made up of energy
waves which are grouped
together in a spectrum.
• Our eyes can only see the
visible light spectrum.
The Visible Light Spectrum
Watch the following:
https://www.youtube.com/watch?v=
pvC9MQvqHMQ
The Colour Theories
• All the colours of the rainbow can be
made from red, yellow and blue.
• So why do we see more colours than
that?
• It has to do with the mixing and taking
away of colours, otherwise known as
Additive and Subtractive Colour
Theories.
Additive Colour Theory
Additive Colour Theory
• The additive color theory
involves light emitted directly
from a source (ex. a television)
• When various amounts of red,
green, and blue light are added
together they produce other
colours.
Additive Colour Theory
• If you add the primary colours of light
together, you can reliably make
secondary colours.
Additive Colours
• Primary Colours • Secondary Colours
– Red (R)
– Green (G)
– Blue (B)
– Yellow (Y)
– Magenta (M)
– Cyan (C)
Additive Colour Theory
• When all of the primary colours are
added together they make white
light.
• This is why the
middle of the
picture below
is white.
Visualize Additive Colour
• Imagine three spotlights, one red, one
green and one blue are shone on a skater.
Where the blue and green spotlights
overlap, the color cyan is produced;
where the blue and red spotlights
overlap, the color magenta is produced;
where the red and green spotlights
overlap the color yellow is produced.
When added together, red, green and
blue lights produce what we perceive as
white light.
Complimentary Colours
• Complimentary colours are any two
colors that when added together,
produce white light.
What are the complimentary colours here?
Red and Cyan
Green and
Magenta
Blue and
Yellow
Additive Colour Theory
Watch the following:
https://www.youtube.com/watch?v=t8ifzy-FSC8
Stop when it starts talking about subtractive colour theory.
OR do the demonstration.
Applications: Additive Colour
• Television is based on additive colour
theory.
• So is theatre lighting!
Applications: Additive Colour
• Television screens and computer monitors:
– Thousands of red, green and blue phosphor dots
make up the images on video monitors.
– The phosphor dots emit light when activated
electronically, and it is the combination of
different intensities of red, green and blue
phosphor dots that produces all the colors on a
video monitor.
– Because the dots are so small and close together,
we do not see them individually, but see the
colors formed by the mixture of light.
In Summary
• Additive color involves the use of
colored lights. It starts with
darkness and mixes red, green, and
blue light together to produce
other colors. When combined, the
additive primary colors produce
the appearance of white.
Homework Questions:
What combination of colours would you
need to:
1. Set a light blue “mood light” on an actor
in a tranquil theatre scene?
2. Cast a yellow “daytime” light in your
home?
3. Make a white light if you only had a cyan
filter?
4. Make a white light if you only had a green
filter?
5. Turn magenta into blue?
6. Turn green into yellow?
7. Turn yellow into red?
Subtractive Colour Theory
• Subtractive colour theory says that if
you remove primary colours from white
light, you can reliably make other
colours.
• This is the way we “normally” think of
mixing colours!
17 February 2019
Subtractive Colour Theory
• Primary colours are subtracted when
they are absorbed by a surface.
• What is reflected by the surface is the
colour we see.
2/17/2019
For example: an apple really has no color; it
emits no light energy of its own. An apple
reflects the wavelengths of light that cause us to
see red and absorbs the other wavelengths,
which makes us see red.
Subtractive Colour Theory
• Subtractive color starts with an object
(such as paper or canvas) that reflects
light.
• Colorants (such as pigments or dyes) on
the object reflect certain wavelengths of
light.
• The wavelengths that are absorbed into
the object are “subtracted”. We see
what is reflected.
Subtractive Colour Theory
• If an object reflects all of the white
light, it appears white.
• If an object absorbs (subtracts) all the
light, it appears black.
Subtractive Colour Pigments
• Pigments are dyes or paints that absorb
different colours:
– For example, a red car absorbs green and
blue, and reflects red.
• Y+C=G
• Y+M=R
• C+M=B
Subtractive Colour Filters
Filters are transparent, allowing light to pass through
them. Some light is absorbed by the surface below
the filter, and some is reflected.
•
Primary Colours:
–
–
–
•
Secondary Colours
–
–
–
•
Red (R) Filter = Absorbs G - B
Green (G) = R - B
Blue (B) = R – G
Magenta (M) = G
Cyan (C) = R
Yellow (Y) = B
Tints and Shades:
–
–
White: Clear Filter
Black: Opaque Filter
Subtractive Colour Theory
• Watch the following:
https://www.youtube.com/watch?v=t8ifzy-FSC8
Begin from where you left off – when it starts talking about
subtractive colour theory.
Applications: Subtractive Colour
• Printers use color inks that act as filters and
subtract portions of the white light.
• Printing inks are transparent, allowing light to
pass through to and reflect off of the paper.
• The printing process uses cyan, magenta and
yellow inks, and a fourth ink, black.
• Each process printing ink (cyan, magenta,
yellow) absorbs or subtracts certain portions
of white light and reflects other portions back
to the viewer
Applications: Subtractive Colour
•
•
•
•
•
Paints
Clothing and food dyes
Photography
Newspapers, magazines
Printers
In Summary
• Subtractive color involves colorants
and reflected light. It uses cyan,
magenta and yellow pigments or
dyes to subtract portions of white
light illuminating an object to
produce other colors. When
combined in equal amounts, pure
subtractive primary colors produce
the appearance of black.
2/17/2019
Homework Questions
Suppose you’re working on a newspaper,
and are printing the centre spread. What
would you need to do to produce:
1. The bright red of an apple?
2. The colour of the afternoon sky?
3. The green leaves of a tree?
4. The deep blue of a dragonfly’s body?
5. The white of snow?
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