Unit #3 - Optics Chapter 11.3 - Colour Theory Additive Colour Theory of Light white light is composed of different colours (wavelengths) of light. It is possible to produce white light by combining only three colours ‣ red, green, and blue (primary colours) white light green light red light blue light magenta yellow cyan Figure 10.18 (a) All three primary colours together produce white light. (b)The three primary colours of light are red, green, and blue. When paired, they can create three secondary colours: magenta, yellow, and cyan. (a) (b) Primary Colours ‣ cannot be formed by other colours Secondary Colours ‣ formed by two primary colours retina optic nerve blood vessels focal point cornea iris pupil lens ciliary muscle The Human Eye Detecting Light photoreceptors are cells in the retina which are sensitive to light ‣ Rods - detect shapes and movement in LOW light; no colour ‣ Cones - detect colours (Red, Green, Blue) Blind Spot - where the optic nerve attaches to the retina.
Transcript
Unit #3 - OpticsChapter 11.3 - Colour Theory
Additive Colour Theory of Light
white light is composed of different colours (wavelengths) of light.
It is possible to produce white light by combining only three colours‣ red, green, and blue (primary colours)
Additive Colour Theory of LightThe additive colour theory of light states that white light is composedof different colours (wavelengths) of light. It is possible to producewhite light by combining only three colours. One such combination isred, green, and blue. These three colours of light are known as primarycolours. If you mix correct amounts of all three primary colours of light,you will make white light (Figure 10.18(a)). If you mix only two of theprimary colours together, you will make a secondary colour. Thesecondary colours of light for red, green, and blue are magenta, yellow,and cyan as shown in Figure 10.18(b).
387Light is part of the electromagnetic spectrum and travels in waves.
wavelength (nm)
700
600
580
550
450
400
radiowaves
micro-waves
infra-red
ultra-violet
X-rays
gammarays
visiblelight
Figure 10.17 The visible spectrum has red light at one end, violet light at the other end, andall the other colours in between. Red light has a relatively long wavelength of 700 nm(nanometres) while violet light has a shorter wavelength of about 400 nm. A nanometre is one-billionth of a metre, so 700 nm is 0.000 000 7 m.
white light
green light
red light
blue light
magenta
yellow
cyan
Figure 10.18 (a) All three primary colours together produce white light. (b)The three primarycolours of light are red, green, and blue. When paired, they can create three secondarycolours: magenta, yellow, and cyan.
(a) (b)
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Primary Colours
‣ cannot be formed by other colours
Secondary Colours
‣ formed by two primary colours
Human VisionIn order to evaluate optical technologies related to the human perceptionof light, it can be helpful to understand how your own eyes work tofocus light and detect images (Figure 12.6).
The outer surface of your eye where light enters is made of atransparent layer of tissue called the cornea. Light can pass rightthrough the cornea because even though it is made of living cells, it iscompletely clear. Your cornea is made of strong tissue that is toughenough to protect your eye and hold it together, while remainingextremely sensitive to touch. The cornea is about as thick as a creditcard and is sensitive enough to send you a strong pain signal if anythingtouches it. If it suffers from a small scratch, the cornea can heal itself.The light rays that arrive at your eye are refracted by the cornea. This helps direct the light correctly into your eye. Without therefractive properties of your cornea, you would not be able to focus.
After passing through the cornea, the light rays reach the pupil. Thepupil is the dark circle that you see when you look at someone’s eye. Itis actually just a hole that allows light to pass into the eye. The pupil isblack for exactly the same reason the entrance to a cave appears dark —light rays enter the cave but do not leave. The pupil is created by acircular band of muscle called the iris. When people refer to their eyecolour, they are referring to the colour of the iris. The iris controls thesize of the pupil, and so it controls the amount of light that enters theeye. In dim light, the iris opens and the pupil dilates (becomes wider)to let in more light. In bright light, the iris closes and the pupil contracts(becomes smaller) so that less light enters (Figure 12.7). Changes inpupil size happen automatically; you do not have to think about it.
470 UNIT D Light and Geometric Optics
retina
optic nerve
blood vessels
focal pointcornea
iris
pupil
lens
ciliarymuscle
Figure 12.6 A cross-section of the human eye
Figure 12.7 (a) Dilated pupil and (b) contracted pupil
WORDS MATTER
“Pupil” is derived from the Latinword pupa, meaning little doll,indicating the tiny reflections ofpeople visible in pupils. The word“iris,” the name of the structure thatdetermines eye colour, is derivedfrom the Greek word for rainbow.
(a)
(b)
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The Human Eye Detecting Light
photoreceptors are cells in the retina which are sensitive to light‣ Rods - detect shapes and movement in LOW light;
no colour‣ Cones - detect colours (Red, Green, Blue)
Blind Spot - where the optic nerve attaches to the retina.
Detecting Light
Detecting LightGetting light into the eye and focussing it on the retina are only part ofthe task of seeing an image. In order for you to see, light rays must beabsorbed by photoreceptors, which are cells in the retina that aresensitive to light. Photoreceptors include rod cells and cone cells (Figure12.10). Rod cells help us detect shapes and movement in low lightsituations. Our brain does not recognize differences in colour fromsignals gathered by rod cells. Instead, we detect only shades of grey.Most of us are so used to our low light vision abilities that we do noteven notice that we are not seeing in colour. Cone cells arephotoreceptor cells used to detect colour. In humans, cone cells come inthree types, each of which detects a different primary colour of red,green, or blue particularly well.
There is one place on the retina of every healthy eye called the blindspot, which has no photoreceptors and which cannot detect light. The blind spot is the place where the optic nerve attaches to the retina.The optic nerve connects your eye to your brain. You do not noticeyour blind spot because your brain “fills in” that spot with whatevercolours are nearby in what you are looking at. You can use Figure 12.11to help you detect your blind spot.
472 UNIT D Light and Geometric Optics
Suggested Activity •D28 Inquiry Activity on page 478
retina
blind spot
<photo to come>
Figure 12.10 This is a false-colour electron micrograph image showing rod and cone photoreceptor cells in the retina at amagnification of 1800! at 10 cm. Cones are found in the central region of the retina. The more numerous rods are locatedoutside the central region of the retina.
+ •Figure 12.11 To find the blind spot in your right eye, close your left eye, and stare at the plussign. Slowly move the book toward you and away from you. When the black spot disappears,you have found your blind spot.
rod cell
cone cell
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Detecting LightGetting light into the eye and focussing it on the retina are only part ofthe task of seeing an image. In order for you to see, light rays must beabsorbed by photoreceptors, which are cells in the retina that aresensitive to light. Photoreceptors include rod cells and cone cells (Figure12.10). Rod cells help us detect shapes and movement in low lightsituations. Our brain does not recognize differences in colour fromsignals gathered by rod cells. Instead, we detect only shades of grey.Most of us are so used to our low light vision abilities that we do noteven notice that we are not seeing in colour. Cone cells arephotoreceptor cells used to detect colour. In humans, cone cells come inthree types, each of which detects a different primary colour of red,green, or blue particularly well.
There is one place on the retina of every healthy eye called the blindspot, which has no photoreceptors and which cannot detect light. The blind spot is the place where the optic nerve attaches to the retina.The optic nerve connects your eye to your brain. You do not noticeyour blind spot because your brain “fills in” that spot with whatevercolours are nearby in what you are looking at. You can use Figure 12.11to help you detect your blind spot.
472 UNIT D Light and Geometric Optics
Suggested Activity •D28 Inquiry Activity on page 478
retina
blind spot
<photo to come>
Figure 12.10 This is a false-colour electron micrograph image showing rod and cone photoreceptor cells in the retina at amagnification of 1800! at 10 cm. Cones are found in the central region of the retina. The more numerous rods are locatedoutside the central region of the retina.
+ •Figure 12.11 To find the blind spot in your right eye, close your left eye, and stare at the plussign. Slowly move the book toward you and away from you. When the black spot disappears,you have found your blind spot.
rod cell
cone cell
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Blind Spot - where the optic nerve attaches to the retina.
