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Lecture Notes - 6/24/99
Lera Boroditsky

Light & Eyes: Lecture Notes

0. Introduction



I. Light
  • 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 landing stripes.

      • 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



II. Eyes
  • 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 vision).

    • 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.

      • 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 pit.

      • 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.


  • Optics

    • 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 close objects.

        • 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


  • Retina

    • photoreceptors

      • [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 light.

      • [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

      • 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 membrane.

      • 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 37]

      • 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 photons.

      • 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 bleaching.

      • 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 level.