PSYC 212 Chapter 8 1
A. The Nature of Light
Nearly one half of the brain is involved in processing visual information.
For centuries, it was believed that light was made up of particles emitted by certain objects,
such as the Sun or a candle flame, which travelled through the ether in straight lines.
Around the mid-17 century, this particle theory was questioned, and a wave theory
emerged. In the early 1800s, it became clear that the particle theory did not explain certain
All waves must necessarily occur as a deformation of a medium.
What the medium is was unknown. But the ether (medium) must be rigid so as to permit
light to travel through it at such high speeds.
This ether was believed to be in space, as well. But it didn’t make sense how planets could
move through this rigid medium so easily. The particle theory began to make sense again.
In the 20 century, it was found that light beams produce an ejection of electros from an
object. This could only happen if light consisted of particles.
The idea that light travelled in photons (small packets) that also had wave-like qualities
emerged. This lead to the dual-theory of light (current understanding of the nature of light).
1. General Properties of Light
James Clark Maxwell (1873) discovered electromagnetic radiation: characterized by the
flow of photons.
The movement of photons through space produces an oscillating electric and magnetic field
that together move in the same direction as the photon (mutually linked, varying intensity).
The fluctuating electric and magnetic field essentially represents the waves (since they are
perpendicular, it is sufficient to just show one).
Visible Light is an Example of Electromagnetic Radiation
The spectrum of electromagnetic radiation is truly impressive (vast).
Long-wavelength radiation can be measured in meters, and short-wavelength can be
measured in nanometers (with different units used in between).
Shortest to longest wavelengths:
- Gamma rays (10 -10 nm)
- X-rays (10 -10 nm)
- Ultraviolet rays (10 -10 nm)
- VISIBLE LIGHT (400-700 nm)
- Infrared rays (10 -10 nm)
- Radar waves (10 -10 nm)
- Microwaves (10 -10 nm)
- FM radio (10 -10 nm)
- Television (10 -10 nm)
- AM radio (10 -10 nm)
- ELF waves (10 nm and up)
Only a very small segment of the spectrum is used for sensory function.
Wavelengths correspond with perceived color:
- Red = 700 nm
- Orange = 650 nm
- Yellow = 600 nm
- Green = 550 nm
- Light Blue = 500 nm
- Dark Blue = 450 nm PSYC 212 Chapter 8 1
- Purple = 400 nm
The emission spectrum of the Sun is restricted to 100-4000 nm.
The transmission spectrum of the atmosphere has a similar profile to the Sun’s emission
spectrum and overlaps it within the same wavelength range.
Speed of Light
Light is transmitted along straight lines at an amazing speed.
The first attempts to measure the speed of light occurred in the late 1600s.
The speed of light in vacuum is 299,792 kilometers per second.
Light energy can travel in the absence of a medium, such as in outer space.
When travelling through a denser medium, such as air or glass, the speed of light decreases.
To best understand how light spreads out, it is convenient to symbolize it with an imaginary
point (point source). See Figure 8.5 on page 250.
Wave fronts are small in the immediate vicinity of the point source, and grow as they
At about 6 meters from the source (optical infinity), the wave fronts become so large that
they are essentially flat.
Light rays are more easily represented. These are represented as straight lines radiating
from the point source.
Effect of Distance on Light Intensity
Because the rays diverge more the farther away from the source they are, the intensity
decreases (a light is dimmer if you are farther away).
This is not linear, but is related to the square of the distance:
- The light rays pass through 1d;
- The rays will diverge enough to pass through 4 times the area at the distance of 2d;
- The rays will diverge enough to pass through 9 times the area at the distance of 3d.
The total number of photons per unit area decreases with increased distance.
Inverse square law = intensity is inversely proportional to the square of the distance.
2. Interaction of Light with Objects
Absorption, Reflection, and Transmission
Absorption: when atoms can convert the energy contained in light into vibrational motion.
Reflection: atoms fail to convert the energy and re-emit it back as reflected light.
In transparent objects, light can simply pass through (transmission).
Perceived colors: an object may only reflect certain wavelengths while absorbing others.
Objects that reflect all wavelengths will be perceived as white.
Occurs when light interacts with gaseous particles.
Light is first absorbed by a particle, and then re-emitted in a random direction.
Rayleigh scattering: particles are very small (like nitrogen or oxygen); is the most common
form above 4.5 km in the atmosphere; short-wavelength light is scattered more than long-
wavelength = this is the reason the sky looks blue (short-wavelength) and why the horizon
at during dusk/dawn is red (long-wavelength).
Mie scattering: large particles such as dust, some and water vapor; only occurs below 4.5
km in the atmosphere; not wavelength-dependent; occurs in the direction of light
propagation; why we see a white glare around the Sun.
Non-selective scattering: particles are much larger than the wavelength of light; not
wavelength-dependent; why clouds appear white (the water droplets and ice crystals
contained in them produce a non-selective scattering of light). PSYC 212 Chapter 8 1
Recall the electromagnetic field.
Plane-polarized light: when the electric (and magnetic) fields are restricted to only one
place (vertical, horizontal, etc.); this only occurs under certain conditions.
Unpolarized light: more common; there is array of electric (and magnetic) fields in all
possible orientations/infinite planes (see figure 8.8 page 253); sunlight and nearly all
3 common ways that polarization take place:
- Light being scattered by a particle;
- Transmission through certain crystals that absorb more light in one plane than
3. Basic Principles of Optics
Eye has two essential functions:
- Form an image of the outside world upon the retina;
- Transduce the light energy in that image into neural signals.
The bending of light that occurs when it travels from one medium to another.
Due to a change in speed as light moves between media.
The refractive index can help indicate the speed of light in a particular medium.
The greater the refractive index of a medium, the slower the speed of light within it.
3 important properties of refraction:
- The medium must be transparent;
- The greater the difference in refractive index between the two media, the greater the
- Refraction will only take place if an incident light ray strikes the boundry at an angle
away from the normal (an imaginary line that is perpendicular to the boundary
separating the media).
Divergence and Convergence of Light by a Lens
Concave lenses = divergence, two surfaces bowed inward, like this: ) (
Convex lenses = convergence, two surfaces bowed outward, like this: ( )
See Figure 8.12 on page 255.
The optic axis is an imaginary line connecting the midpoints of the two refracting surfaces.
Lens PowerAffects Image Location
The refractive power of a lens is determined by how much refraction will occur at its two
Modifying the curvatures will alter the refractive power.
Greater surface curvature = greater refracting power (and am image closer to the lens).
Object DistanceAffects Image Location
As an object moves closer (not at optical infinity), the image distance increases.
See Figure 8.14 on page 256.
B. The Human Eye
1. Structure of the Eye
Peripheral Components of the Eye
The eyeball is a nearly spherical object about 1 inch in diameter.
Sclera = tough outer membrane. PSYC 212 Chapter 8 1
Cornea = transparent front part of the eye that serves as the first refractive element.
Choroid = next layer, very rich blood supply.
Retina = “film of the eye” where an optical image is created on the back wall of the eye.
Neurons transduce the light energy of the image into a set of neural signals, which are
carried in nerve fibers (optic nerve), which outputs to the brain.
Internal Components of the Eye
Aqueous humour = liquid, immediately behind the cornea.
Vitreous humour = gelatinous, in front of the retina.
Crystalline lens = separates the humours, often just called the “lens,” held in place by small
ligaments, serves as the second refractive element.
Iris = right in front of the lens, colored.
Pupil = hole in the center of the iris that allows light to proceed into the lens, size in
controlled by the iris.
2. Optical Properties of the Eye
The Emmetropic Eye
Schematic eye = a theoretical model that takes into account the optical properties of the
cornea and lens to arrive at details of image location and quality.
Two-thirds of refraction occurs at the air-cornea boundary, while only one-third actually
occurs in the lens. This is because of the greater difference in the refractive index.
An eye is said to be emmetropic as long as the requirement that incoming parallel light be
brought to a sharp focus on the retina is met.
The Problem of Near Distances
Near objects produce a corresponding backward shift in the image.
Technically, the image shift of a near object would go behind the retina.
This would appear as a blur on the retina.
The Process ofAccommodation
The eye changes the curvature of its lens to focus on near objects (coined accommodation).
The blur stimulates accommodation in the ciliary muscles.
Constriction = more rounded surfaces of the lens.
There is a limit to this constriction though, and this usually occurs at 12 cm for most young
people (this is called the near point).
Anything closer will appear blurred.
Involuntary reflex, automatic.
3. The Retinal Image
Extended Objects and their Retinal Image
Think of a complex visual scene as having multiple point sources.
Each point source emits light, which is refracted by the optical elements of the eye.
This produces a corresponding retinal image point.
Even the most complex visual scene can be broken down into a simple series of point
The rays from the point sources can be easily traced through a schematic eye to define the
resulting retinal image.
Optical Transformations in the Retinal Image
The image is transformed in two ways:
- First, the image is inverted (the retinal image is upside down);
- Second, the image is horizonta