Psychology 2115A/B Lecture Notes - Color Vision, Electromagnetic Spectrum, Trichromacy

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Published on 20 Apr 2013
School
Western University
Department
Psychology
Course
Psychology 2115A/B
Chapter 9 Review: Seeing Colour
“… The Rays to speak properly are not coloured. In them there is nothing else than a certain Power and
Disposition to stir up a Sensation of this or that Colour” – Sir Isaac Newton
Colour doesn’t exist – the only place it exists is in your head strictly psychological
We have detectors in our body that can detect a certain kind of wavelengths and we have learned how
to process this and have given it a name
Do Objects Possess The Colour we see? NO!
An object gets its colour form the wavelength of light it reflects
The colour of an object is the wavelengths that is reflecting back to your eye to be detected by the
cones
A red apple is actually absorbing all other wavelengths of light and reflecting back only the red
wavelength to your eye
o White light comes down and hits the red apple all the wavelengths are absorbed except those in
the red-end of the spectrum
o Bounced back into the retina and stimulates certain cones, which makes us perceive red
o It is only when we start pulling out certain wavelengths that aren’t absorbed that we see colour –
everything else is absorbed
Something that is white is reflecting all the wavelengths of colour to your eye
o White light = all of the visible wavelengths together
Something that is black is absorbing all the wavelengths of colour
o No reflectance
Light and Colour
No detector for gamma rays
Visible light spectrum: the tiny spectrum that we can see
Visible light occupies the electromagnetic spectrum from approximately 400-700 nm
o When we are talking about nanometers we are talking about how long a wavelength is the
distance between one full wave
o Lower than 400 = push outside visible light spectrum to ultraviolet light this light reflects but
we cannot see it
o Over 700 = go into infrared not able to see red anymore
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Newton’s Prism Experiments
He showed that, once broken down into a spectrum, single components could not be broken down
further
Thought that the light out there is composed of different colours of light
Passed white light through a prism and could see a rainbow white light is composed of a bunch of
different colours of light
Once you have broken white light into its component pieces, it can’t be broken down any further
He showed that it was possible to recombine several wavelengths to produce white
Can break white light up into all of the colours
If we took a bunch of lights that made all the wavelengths and we shine them all in one spot, we get
white light
White light makes coloured light, coloured light combines to create white light
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What is Colour for?
Colour vision capacity varies a great deal across species
o Primates have trichromatic colour vision like us, but not all animals are like this have different
receptors and therefore see things in different ways than we do
No definitive answer as to why colour vision evolved, but it seems likely that it provided an
advantage in the identification of food sources or in mate selection
o We have developed vision to help us deal with what is in our environment evolutionary
What Colours do we see?
All discriminable colours can bee described in terms of 4 colour names: blue, yellow, green, red
How Many Colours can we see?
Can calculate the theoretical maximum based on the number of JNDs for each colour dimension
Wavelength discrimination = 200 JNDs
o We can only tell about 200 different colours based strictly on wavelength
Saturation = 20 JNDs
Brightness = 500 JNDs
Therefore, total range of possible colours = 200*20*500 = 2 million
o The human retina is able to detect 2 million different colours
Producing Colours
Although hue is determined by wavelength, we are very rarely exposed to single wavelengths
Most of the time, what we see is a mixture of many different wavelengths
Sensation vs. Perception
Our perception of colour is simply the interpretation of wavelengths
o 400-450 nm = violet
o 450-490 nm = blue
o 500-575 nm = green
o 575-590 nm = yellow
o 590-620 nm = orange
o 620-700 nm = red
Reflectance and Transmission
What wavelengths are reflected from objects
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Document Summary

The rays to speak properly are not coloured. In them there is nothing else than a certain power and. Disposition to stir up a sensation of this or that colour sir isaac newton. Colour doesn"t exist the only place it exists is in your head strictly psychological. We have detectors in our body that can detect a certain kind of wavelengths and we have learned how to process this and have given it a name. An object gets its colour form the wavelength of light it reflects. The colour of an object is the wavelengths that is reflecting back to your eye to be detected by the cones. It is only when we start pulling out certain wavelengths that aren"t absorbed that we see colour everything else is absorbed. Something that is white is reflecting all the wavelengths of colour to your eye: white light = all of the visible wavelengths together.

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