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Light and the Eyes
External stimulus for vision G9 p 22
Light is a portion of the electromagnetic spectrum
Name of energy
AC Circuits
AM Radio waves
TV signals
FM signals
Radar waves
Infrared rays
Light
Ultraviolet rays
X-rays
Gamma rays
What is electromagnetic energy? – particles or waves?
Some forms of electromagnetic energy are best conceptualized as waves, e.g., FM radio signals
Some forms are best conceptualized as particles, e.g, X-rays
Light has been conceptualized in both ways – as rays and as waves.
We’ll think of light as rays or as particles - particles of energy streaming from a light source
A single particle will be called a photon.
If you had a strong enough light, one that could shoot out enough photons, you could use
it as a propulsion device.
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Characteristics of Light in particle terms
Intensity
We’ll conceptualize it as the number of particle streams emanating from the light
source. The more streams, the more intense the light. Note that this is a characteristic of
a collection of streams, not of a given stream.
Wavelength
The distance between particles within a given stream.
Long wavelength – long distance between particles: .
.
.
.
.
.
Short wavelength – short distance between particles: . . . . . . . . . . . . . . .
Wavelength is measured in nanometers (nm).
1 nm = 1 billionth of a meter: .000000001
Visible light: 350 nanometers to 700+ nanometers.
Speed of particles:
Speed of light, i.e., about 300,000,000 (300 million) meters / second.
186,000 miles/second.
(Action potentials travel down the axon at 120 meters / second.)
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Common terms associated with light sources
Lumen. The overall intensity of a light. Roughly: number of particle streams emitted.
Wattage. Amount of energy used. In the old days, itsed to correspond closely to intensity.
800 lumens
15 watts
6000 hours
LED lights, now becoming popular, will provide the same light intensity in lumens with even
less energy consumption and even longer life. (We’ll bequeath our light bulbs to our heirs.)
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The Light Spectrum. A graph representing light intensity and wavelength. G9 p 23
The intensity (no. of particle streams) of light at each wavelength is plotted.
Rough correspondence of spectrum horizontal axis to perceived colors. (Not a spectrum.)
300
400
500
Wavelength in nm
600
700
A. Spectrum of a red monochromatic light – light with only one wavelength.
300
400
500
Wavelength in nm
600
700
600
700
B. Spectrum of light from a regular incandescent bulb.
300
400
500
Wavelength in nm
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C. Rough spectrum of light from a fluorescent bulb.
300
500
Wavelength in nm
600
700
500
Wavelength in nm
600
700
600
700
D. Light from an infrared remote control.
300
400
E. Light from a tanning bed - energy is primarily ultraviolet
300
400
500
Wavelength in nm
F. Result of prolonged exposure to tanning bed radiation.
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Why do we have specific receptors for light, rather than some other part of
the eletromagnetic spectrum?
Why are we sensitive to light, as opposed to gamma rays, or x-rays, or radar waves, or TV waves
or wi-fi waves?
Two main reasons. (These could be test questions – multiple choice or essay.)
1. There’s much more light available to us than other kinds of energy that receptors for it
are easier to build.
Generally speaking, it’s easier to sense light than it is to sense other less abundant energy. . .
a) Individual receptors don’t have to be as sensitive if there is a lot of the energy out there.
We don’t need Hubble telescope receptors if what they’re supposed to receive is a strong signal.
b) There is less need for multiple receptors to maximize sensitivity. We don’t need as many
receptors as there would have to be if the energy were less prevalent.
2. Light is differentially reflected and absorbed from the various objects in the
environment so receptors for light can be used to tell the difference between things.
Long wavelengths such as radio waves are ALL reflected from everything.
So all objects would reflect about the same amount of radiation and so all objects would appear
about the same. Like having a friend who likes EVERYTHING.
Short wavelengths such as cosmic rays, are ALL absorbed by almost everything. So all
objects would be essentially invisible. Like having a friend who hates EVERYTHING.
But some objects absorb short wavelengths of light and reflect long wavelengths of light.
They appear blue.
Some objects reflect short wavelengths and absorb long wavelengths. They appear red.
This is like having a friend who recommends what you would like and does not recommend what
you would not like. So light is like a trusted friend.
Of course, we have to have the neural apparatus to make use of the different wavelengths
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The eye G9 p 23
Two Functional systems in eye
The optical system of the eye
The Cornea
The Aqueous humor
The iris/pupil
The lens
The Vitreous humor
The neural system of the eye
The receptors
The rods
The cones
The bipolar cells
The ganglion cells
The horizontal cells
The amacrin cells
The optic nerve
This lecture
Next lecture
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The optical system G9 p. 23
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The need for focusing –
Needed: A system that creates an in-focus projection onto the back of the eye of the image of
each thing in the external environment.
Solution #1. A wide opening into the eye. (Won’t work.)
Light Source
Projection
“screen”
Wide
opening
Image
Bright
but
blurred
Many light rays enter,
but each strikes a
different spot on the
receiving screen.
Solution #2. A pinhole opening into the eye. Works only in bright bright light.
Pinhole
opening
Only those rays right
on target get through
Image
In
focus
but dim
Rays not on target are
deflected.
Solution #3 – the solution represented by our visual system – wide opening with a lens.
Wide
opening
with lens
All rays are allowed
through and bent so
they all strike the same
spot.
Image
In
focus
and
bright
The rays that go through the center of the lens will be projected “best”.
The rays that go through the periphery will usually be slightly out-of-focus causing the whole
scene to be very slightly blurry.
This fact is important when the function of the iris/pupil is considered.
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The human dual eye focusing system – cornea and lens – G9 p 24
The cornea: Performs most (80%) of the focusing, i.e., “bending” of light rays.
But the cornea’s focusing is fixed – it’s the same for light from near objects as it is from
far objects.
Only objects at one distance will be in focus. All others will be out of focus.
The lens (pretend that the cornea is not in the system for these figures)
Correct Focusing with a fixed lens. Objects whose distance from lens is exactly equal to focal
length will be in focus.
Projection “screen” –
The retina
All light from the same
point in space is
projected to same point
on screen.
Object
Pointbeing
on
viewed
Object
being
viewed.
Problem with a fixed lens: Viewing near objects. Rays from an object whose distance is too
small will focus behind the receiving screen. For proper focus the light rays would have to be
bent more than the lens is constructed for.
Image blurred on
receiving screen
Point on near
object being
viewed
Problem with a fixed lens: Viewing far objects. An object whose distance is too large will
focus in front of the receiving screen. The light rays are being bent too much by the lens.
Image blurred on
receiving screen
Point on far
object being
viewed
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Two possible solutions to the “fixed lens” problem . . .
1) Solution 1- Movable lens: Move the lens either toward the object or away from the object..
This is what is done in cameras and cephalopods (such as octopuses)
2) Solution 2 - Variable thickness lens: Adjust the thickness of the lens.
Lens kept in same
position, but made
flatter for far objects.
Lens kept in same
position, but made
fatter for near objects.
The second solution is what has been adopted for the human eye.
Although the cornea cannot change its shape, the lens, located immediately behind the cornea
does change its thickness.
The ciliary muscles attached to the lens expand and contract to
make it change its shape.
This leads to a concept:
Accommodation: Automatic changes in the shape of the lens to keep objects at different
distances in focus. G9 p 24
The lens automatically becomes flatter when we focus on objects that are far away.
It automatically becomes fatter when we focus on objects that are close to us.
The focusing process is an automatic neural process. The only conscious control over the
mechanism is to indirectly control it by changing the distance of objects we are attending to.
So if I had kidnapped you and tied you to a chair in a basement demanding that you “Make your
lens fat!! Now!!!” what could you do?
Accommodation is accomplished by muscles attached to the lens which contract or expand under
control of signals from a specific collection of neurons designed to perform the focusing
function.
Brain Modules. Note that there is a concept being introduced here – the concept of brain
modules, specific collections of neurons designed to perform certain functions
autonomously. There are multiple brain modules performing multiple functions without
cognitive guidance all the time.
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Accomodation problems: The opias –
Myopia – Near sightedness – you can see best only close-up.
People with myopia can see near objects well, but not far away objects.
The reason: 1) Lens is too fat, so it bends light too much for far objects or
2) eyeball is too long
Far
Object
Light focuses in front of retina
Solution is to wear glasses which “unbend” the light increases the visual angle of objects.
Hyperopia – Far sightedness (hyper – a lot of, e.g., hyperactive)
People with hyperopia can see far objects well, but can’t see near object.
The reason: 1) Lens is too flat, so it doesn’t bend light enough for near objects or
2) eyeball is too short
Near
Object
Light rays focus in back of retina
Solution is glasses which “prebend” the light.
Presbyopia – Old age farsightedness
As we get older, the lens continues to add layers.
Interior layers become less flexible, and lens becomes less able to change shape, staying too flat;
This results in far sightedness.
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The iris
The iris is a ring of muscles that form a circle, like a doughnut, with a hole in the middle.
The hole is called the pupil.
Relation and contraction of the muscles change the size of the pupil.
The tissue of the iris contains pigment that gives the eye its color.
The fine pattern of detail on the iris is determined by a combination of factors that are unique to
the individual eye.
No two irises are alike, not even your left and right iris.
From Daugman, J. Iris Recognition. American Scientist, 89, 326-333.
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The pupil.
Pupil diameter varies from 2 to 8 mm in diameter. Using the formula, п*r2, (that’s pi times
radius squared) this means that the area varies from 3.14*12 to 3.14*42 = 3.14 mm2 to 50 mm2,
about a 17:1 ratio. Who said a psychology major wouldn’t need math?
Thus the pupil admits about 17 times as much light when it is at its widest than it does when it is
at its narrowest.
As the ambient light increases, the iris automatically relaxes, allowing the
pupil to become smaller.
As the ambient light decreases, the iris automatically contracts, making the
pupil larger.
So if I kidnapped you and tied you to a chair in a basement demanding “Make
your pupil bigger!! Now!!” what could you do?
Why not have a fixed-size hole?
Two possible answers . . .
1. The light-regulation hypothesis: To increase the amount of light allowed into the eye when
it’s dark outside and to protect the eye from too much light when it’s very bright outside.
2. The all-things-in-focus hypothesis: To keep the light rays that enter the eye as close to the
center of the lens as possible, making the focus of those rays on the retina as good as possible.
So the pupil is kept small if there’s enough light. It is enlarged only when there isn’t enough
light.
Test question alert!!
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The all-things-in-focus hypothesis – the consequence of a small and large pupil sizes
The following figure is from Blake &Sekular, p. 44, Figure 2.12.
Note that in both figures, the flowers in the foreground are in good focus.
But when viewed through a large opening the background is out-of-focus.
This situation is called poor depth of field by photographers.
When viewed through a small opening, the background is in focus.
This is called good depth of field.
Large pupil
Small pupil
The smaller the opening through which the light passes, the better the depth of field.
Many believe that the primary purpose of the variable-sized pupil is to maximize depth of field.
In addition, with a large opening, there is greater likelihood that “stray” light rays, improperly
focused by impurities in the lens will cause the image to appear less sharp than it would be if
those strays were blocked.
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