Light
Light
& the human eye
What is light?
Light is electromagnetic radiation.
What is light?
Light is electromagnetic radiation.
i
The entire spectrum of
electromagnetic radiation
(EM) includes radio waves
and both gamma rays and
cosmic rays, which have
very short wavelengths
What is light?
Light is electromagnetic radiation.
i
Quantum theory suggests
that light consists of very
small bundles of energy/
particles, a.k.a. photons
i
The entire spectrum of
electromagnetic radiation
(EM) includes radio waves
and both gamma rays and
cosmic rays, which have
very short wavelengths
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
LONG wavelength
LOW frequency
Short wavelength
High frequency
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
492577nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
492577nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
577597nm
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
492577nm
597620nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
577597nm
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
492577nm
597620nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
577597nm
620700nm
Visible light
The wavelength at which these particles move determines the energy and type of EM radiation.
400450nm
492577nm
597620nm
LONG wavelength
LOW frequency
Short wavelength
High frequency
455
492nm
577597nm
i
Visible light is only part
of the EM range, found
towards the middle of the
spectrum - from about
400 nm to 700 nm
...or from violet to red
620700nm
The human eye
‘Visible light’ is defined by
what the human eye can see.
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
i
Rods are sensitive to the
intensity of light
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
i
Rods are sensitive to the
intensity of light
Cones are sensitive to
wavelengths - or colours
The human eye
The retina is lined with light
sensing cells known as
rods and cones.
i
Rods are sensitive to the
intensity of light
Cones are sensitive to
wavelengths - or colours
Different cones are sensitive
to red, green and blue
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
Sensitivity
The ‘sensitivity’ curve
400
460 490 500 530
Wavelength (nm)
600
650
700
The ‘sensitivity’ curve
Blue receptors
Sensitivity
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
400
460 490 500 530
Wavelength (nm)
600
650
700
The ‘sensitivity’ curve
Blue receptors
Green receptors
Sensitivity
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
400
460 490 500 530
Wavelength (nm)
600
650
700
The ‘sensitivity’ curve
Blue receptors
Green receptors
Red receptors
Sensitivity
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
400
460 490 500 530
Wavelength (nm)
600
650
700
The ‘sensitivity’ curve
i
Yellow light (approx. 577-597 nm)
will activate green and red cones
Blue receptors
Green receptors
Red receptors
Sensitivity
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
400
460 490 500 530
Wavelength (nm)
600
650
700
The ‘sensitivity’ curve
i
Yellow light (approx. 577-597 nm)
will activate green and red cones
i
Turquoise light (approx. 470-500 nm)
will activate green, red and blue cones
Blue receptors
Green receptors
Red receptors
Sensitivity
The different sensitivities
of cones allow the brain to
interpret different wavelengths
as different colours.
400
460 490 500 530
Wavelength (nm)
600
650
700
White light
White light
White light
White light
i
White light (which includes all
visible wavelengths) triggers
an electrical message from all
three types of cone
The ‘sensitivity’ curve
The three colour channels of the human eye red, green and blue - enable the brain
to ‘see’ the full visible spectrum...
The ‘sensitivity’ curve
The three colour channels of the human eye red, green and blue - enable the brain
to ‘see’ the full visible spectrum...
The ‘sensitivity’ curve
The three colour channels of the human eye red, green and blue - enable the brain
to ‘see’ the full visible spectrum
...and distinguish about
one million colours
Colour channels
Having three independent colour channels
in the retina is known as trichromacy and
this defines what we see.
Colour channels
Having three independent colour channels
in the retina is known as trichromacy and
this defines what we see.
Colour channels
Colour channels
i
Sea mammals generally have only
one channel - this doesn’t mean
they only see one colour but
delivers a limited palette
Colour channels
i
Sea mammals generally have only
one channel - this doesn’t mean
they only see one colour but
delivers a limited palette
Colour channels
i
i
Sea mammals generally have only
one channel - this doesn’t mean
they only see one colour but
delivers a limited palette
Dogs have two channels
so they see the world in
different colours
Colour channels
i
i
Sea mammals generally have only
one channel - this doesn’t mean
they only see one colour but
delivers a limited palette
Dogs have two channels
so they see the world in
different colours
Colour channels
i
i
i
Sea mammals generally have only
one channel - this doesn’t mean
they only see one colour but
delivers a limited palette
Dogs have two channels
so they see the world in
different colours
Fish, birds and insects have four
channels, enabling them to detect
other wavelengths including UV
Why we see different colours
Objects absorb and reflect light in different ways.
The way we ‘see’ them depends on which
wavelengths are absorbed and reflected.
Why we see different colours
Objects absorb and reflect light in different ways.
The way we ‘see’ them depends on which
wavelengths are absorbed and reflected.
Why we see different colours
Objects absorb and reflect light in different ways.
The way we ‘see’ them depends on which
wavelengths are absorbed and reflected.
i
A red apple reflects EM
wavelengths of about
620-700 nm
Why we see different colours
Objects absorb and reflect light in different ways.
The way we ‘see’ them depends on which
wavelengths are absorbed and reflected.
i
A red apple reflects EM
wavelengths of about
620-700 nm
These light waves reach the
retina and trigger signals in
the brain
Why we see different colours
Objects absorb and reflect light in different ways.
The way we ‘see’ them depends on which
wavelengths are absorbed and reflected.
i
A red apple reflects EM
wavelengths of about
620-700 nm
These light waves reach the
retina and trigger signals in
the brain
We see the apple as red
Questions
?
What is light?
Electric magnetism
Electric radiation
Magnetic radiation
Electromagnetic radiation
What is light?
7 Electric magnetism
7 Electric radiation
7 Magnetic radiation
Electromagnetic radiation
What range of wavelengths make up the visible
spectrum for the human eye?
350 – 600
400 – 700
450 – 850
550 – 950
What range of wavelengths make up the visible
spectrum for the human eye?
7 350 – 600
400 – 700
7 450 – 850
7 550 – 950
What does white light consist of?
One single wavelength
All visible wavelengths
No visible wavelengths
Wavelengths outside the visible spectrum
What does white light consist of?
7 One single wavelength
All visible wavelengths
7 No visible wavelengths
7 Wavelengths outside the visible spectrum
What are the colour ‘channels’ of the human eye?
Green, blue, red
Blue, orange, black
Green, blue, white
Blue, red, yellow
What are the colour ‘channels’ of the human eye?
Green, blue, red
7 Blue, orange, black
7 Green, blue, white
7 Blue, red, yellow
How many colour channels do dogs see?
1
2
3
4
How many colour channels do dogs see?
7 1
2
7 3
7 4
Thank you
Thank you
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