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Lecture Notes - 6/24/99 Light & Eyes: Lecture Notes
- Early philosophers (Plato and Euclid included) thought that vision
was accomplished not by light entering an eye, but rather by particles
shooting out of the eyes to explore surrounding objects (like
- [Figure 1] Vision begins when light
comes into the eye. Light is focused by the cornea and the lens onto
the retina, a thin layer of neural tissue at the back of the eye which
contains photoreceptors. Photoreceptors transduce light into neural
signals and pass their signals on to the brain.
- What is light? Wave or particle? It's both. The smallest unit of
light is a photon (as named by Einstein). Photons are emitted when
electrons in an atom jump from one orbit to another.
- Photons travel at a constant speed of about 300,000 km per second
-- the speed of light. The photons absorbed by your photoreceptors when
you look at a star have been travelling for millions or even billions of
years before making it into your eye.
- The wavelength is the distance between
the two peaks of a sinusoidal wave.
- What kind of light do we see?
- We can only see electromagnetic radiation in the range between about 400
and 700 nanometers.
- Although we only see a small portion of the spectrum, 83% of the
light present in our atmosphere is within this range, so we're making
good use of the information available to us
- Other animals can see things we can't.
- Some butterflies see ultraviolet markings on flowers which serve as
- Some snakes can see infra-red waves: even in complete darkness a
python can see a body-heat image of its prey
- Light is changed by the objects it encounters in its path [Figure 7]
- Light is reflected
- Light is absorbed
- Light rays are bent or refracted
- Light is diffracted
- What makes a good eye?
- how many, how good, and where to put them?
- There is a great variety of eyes. Different creatures have
different eyes because the eyes have adapted to the needs and
environment of their owners.
- Prey usually have eyes on the sides of their head so they can see
behind them; predators have eyes in front. Where are your eyes?
- Did you know you can see stuff that's behind your eyes?
- rabbits have eyes spaced so far apart
that they almost have 360 degree vision (what's the compromise?)
- marine animals also have eyes far apart on the sides of their
heads - e.g. whales have a huge blindspot right in front, so they can't
see a boat coming straight at them. Whalers take advantage of this fact
and always approach a whale dead-on so that it can't see them.
- hawks and other birds have amazing visual accuity (as much as 8
times that of humans) (insert amuzing coast-guard anecdote here)
- how to best capture and focus the light?
- the simplest way: the pinhole camera [Figure 16]
- if the hole is too large, you get a fuzzy image
- if the hole is too small, you get diffraction
- with a small hole, you can't see too well in medium or low light
- SO, you need a way to focus the light - that's why we have a cornea
and a lens
- Anatomy of the human eye [Figure 17]
- from the outside (look in the mirror)
- Sclera: white part.
- Pupil: hole.
- Iris: like the aperture on a camera - comes in several
fashionable colors. The iris has to be dark enough to not reflect all
the light incident on the eye (this is why albinos have such poor
- on the inside
- Cornea: thin, transparent covering of the eye ball. Serves
as the chief refracting/focusing element of the eye. About two thirds of
the optical power of your eye is in the cornea and one-third is in the
- Lens: adjustable focus for near/far (more on this later).
- Fovea: part of retina corresponding to central part of
visual field. Fovea is latin for "pit", and it is actually shaped like a
- Optic disc: part of the retina corresponding to blindspot.
- Optic nerve: made up of ganglion cell axons that exit
through the optic disc.
- Anterior chamber/aqueous humor: fluid filled region in front
of the lens.
- Posterior chamber/vitreous humor: fluid (jelly-like) filled
region behind the lens.
- Why don't you see a hole where your blind spot is? How does
it get filled in? [Figure 18]
- Why don't we see the blood vessels in our eyes? It's
because images stabilized on the retina fade. You can see this by
staring at the center dot in this image -
you have to get close and don't move your eyes.
- Glaucoma: the draining of the aqueous humor is blocked and
pressure is bulit up inside the eye which impinges on the blood vessels
and the optic nerve. If
caught early, it can be treated by medication or by surgery.
- Cataract: a clouded lens which, if serious, can be removed
and replaced surgically
- Macular Degeneration: a desease of the retina where the very
center of the visual field (the fovea and close surround) is damaged. A
devastating disease because you can't see anything that you try to look
at - you can only see periphery. It also makes it impossible to read.
- the retina is pink, but when we look into someone's eye, it looks
black - why is that?
- [Figure 20] you can't see the light
reflecting straight off the retina because your head is blocking the
light that would come in
- so we need an ophthalmoscope to see the inside of an eye
- the back of the eye of cats, racoons and many nocturnal
animals has a reflective coating. This is why their eyes shine at
night. The reflective coating is useful for detecting as much light as
possible in low-light conditions.
- why do you get "red-eye" in photos taken with a flash?
- [Figure 23] visual angle: visual
angle is a measure of the amount of the visual field that an object
takes up. Visual angle is not a measure of object size because:
- objects of the same size, but at different distances from the eye
take up different visual angles
- objects of different sizes at a different distances from the eye
can take up the same visual angle
- accomodation: the process of adjusting the lens in your eye for
different viewing distances
- Almost 70% of the optical power of our eyes is accomplished by the
cornea. But the cornea can't adjust, so it's not good for focusing on
objects at different distances.
- [Figure 24] The focusing power of the
lens can be adjusted. The lens has muscles attached to it that change
its shape and focusing power.
- For viewing distant objects we relax the muscles. When the muscles
are relaxed the lens has little curvature (is flat).
- For looking at nearby objects, we adjust the shape of the lens by
pulling the muscles taught so the lens becomes more round. Draw a
diagram for yourself to see why a rounder lens is better for focusing
- the optical power of a lens is just the inverse of the focal
length. Optical power is calculated in diopters (the same measure your
eye-doctor uses when writing prescriptions).
- [Figure 25] accomodation doesn't
work equally well in all people. Some of us have lenses that don't
properly focus distant objects. That's why many of you wear eyeglasses
or contact lenses.
- [Figure 26] the lens gets more
rigid with age. That's why even people who've had perfect vision all
their lives usually need reading glasses by the age of 50 or so.
- chromatic aberration
- our optics are best equipped for light of 580 nm - red
- short-wavelength light (blue) is not well focused by our optics
- blurring: point-spread and line-spread
- light is always blurred at least a
little bit as it passes through the optics
- for any point of light, there is a normal distribution of noise
- [Figure 28] rods and cones
- conveniently, rods are rod-shaped and cones are cone-shaped
- the rods are good for low-light black and white vision
- [Figure 29] the cones only work in
good light and can detect color. The three types of cones are sensitive
to different wavelengths of light: S-cones to short wavelength light,
M-cones to medium wavelength light, and L-cones to long wavelength
- [Figure 30] distribution of
photoreceptors in the retina: the fovea is all cones, but the periphery
is predominantly rods
- this means that you have good color vision in the center of your
vision, but not in the periphery (try the playing card demo)
- this also means that at night, your vision is best not in the
center, but a little bit to the side (stars look brighter if you don't
look directly at them).
- nocturnal animals have mostly rod vision
- light that enters the eye has to travel through other cells in the
retina and most of the legth of the photoreceptors in order to be
captured. why do the photoreceptors face away from the light? one
reason is they need to be close to the pigmented epithelium which
supplies them with important resources
- Transduction is the most important function of the retina.
- Visual transduction is the process by which light energy is
converted to neural energy (or electrical signals) so that it can be
interpreted by the brain.
- When a photon is absorbed by a molecule of rhodopsin, it changes
the chemical state of the photopigment. The two parts of the molecule
split. This change in state is called the isomerization of the
photopigment. The isomerization sets off a biochemical chain reaction
that eventually leads to an electrical current flowing across the
- Once an electrical response is initiated in the photoreceptors,
this signal is relayed to the bipolar cells, which in turn relay it to
ganglion cells, and thus to the brain. [Figure
- Absorbing even a single photon of light in a rod is enough to evoke
a regular/reliable photocurrent. In fact, Hecht, Schlar and Perrine
demonstrated in the 40's that human subjects can reliably detect single
- Check out these Vials of rhodopsin in
different bleached states. Each vial contains a solution of
rhodopsin that has been exposed to a different amount of light. The
rhodopsin pigment changes color when exposed to light. This is called
- The rods all have the same photopigment, rhodopsin. But there are
three different types of cones in the human retina, each with a slightly
different photopigment. In the butterfly
you can actually see the cone photopigments by just shining light into
the eye. In the human retina, the photoreceptors are way too small to
make a picture like this.
- light/dark adaptation [Figure 36]
- Between night and day, light intensity can change dramatically (by a
factor of forty billion).
- by adjusting the pupil size, we can reduce the change in light
intensity on the retina, but the number is still around three billion
- so how can we see in such different light conditions?
- our eyes accomplish this feat by switching off between using rods
and using cones
- rods are good for detecting single photons, while cones never
saturate even at high light levels - together they can cover the full
range of light intensities.
- switching between rods and cones can take a while. You've probably
experienced this at the movie theatre. When you first walk into the
dark theatre, it's hard to see anything, but after a few minutes you can
see a lot more.
- light adaptation is the second most important function of the
retina. Important point: the brain is not interested in absolute
intensities, but rather in intensities relative to the ambient light