eyesight 1 Flashcards
what sensory system is vision?
dominant sensory system, half of the cortex plays a role in processing visual info
light is…
a form of electromagnetic radiation – energy produced by vibrations of electrically charged material – that is visible to our eyes
Dual theory of light:
light is both a wave and a stream of particles (photons)
- Light propagates like a wave
- Light is absorbed like a particle of energy
Photons:
Light energy is concentrated in small packets of energy, which travel through space in a wave-like manner
Electromagnetic spectrum
Visible light is a tiny portion of this spectrum
Light as a wave has a:
Wavelength (represented by the Greek letter lambda, λ):
the distance between two consecutive crests or troughs of a light wave
Different wavelengths produce different perceptual impressions that we call colour (hue) (shortest wavelength = blue = higher energy)(largest wavelength = red = less energy)
Light as a wave has a:
Frequency
(# of waves/s):
The energy content is inversely proportional to its frequency (small waves = lots of energy = more dangerous = gamma x-rays) (large waves = less dangerous = less energy = television, FM radio, microwaves)
Amplitude:
The difference between the baseline and the peak/trough
determines its brightness or intensity
Optics
The study of light rays and their interactions. Light rays travel in straight lines until they interact with things.
Reflection
the bouncing of light rays off a surface. Most of what we see is reflected light
Absorption
the transfer of light energy to a particle or surface. Surfaces that appear black absorb the energy of all wavelengths. Some compounds absorb only a limited range of wavelength
Transmission
when light is neither reflected nor absorbed by a surface it is transmitted
Refraction
the bending of light rays when light travels from one medium to another (ex: things look blurry underwater cause eyes are used to air, weaning goggles prevents this). Occurs because the speed of light differs in the 2 media
Scattering (diffraction)
when light is dispersed in an irregular fashion
Cornea of eye
continuation of sclera (just at front of eye) , transparent, contains sensory endings, refractive, no blood vessels so can cause diffraction
Retina
contains photoreceptors, where light is focused to make an image of the outside world, pass light to optic nerve
Optic Nerve
a bundle of axons that transmits visual info to the brain
Lens
elastic, transparent, refractive, connected to suspensory ligaments (zonules of zinn) that control how flat/fat lens is, and helps bend light rays
Focusing light on the retina
Schematic Eye
2 main refractive components (both bend light rays)
- Cornea (performs most of the refraction)
- Lens (adjustable!) - additional refraction
Focusing light on the retina
Emmetropic (optically normal) eye - everything is working just right:
Refractive power of optical elements are perfectly matched to the length of the eyeball (has effective cornea and lens that bend rays to reflect on back of eye)
- Parallel light rights from an object at “optical infinity”
- i.e., a “point source” > 6 m (20 ft) away (point far away is focused as point on back of retina)
What if an object is closer than optical infinity?
For near objects, lens and cornea don’t have enough refractive power to converge divergent rays onto a point on the retina
- Results in blurry image
Accommodation
a change in the curvature (fatness) of the lens in response to changing object distances
Thicker lens bends light rays more to focus on closer points
When muscle contract the zonules of zinn dont pull as much
close = contract
Contract muscles = lens gets round (muscles get close to center of lens)
- For most young people, limit of accommodation is ~10 cm→ near point (how close we can bring an object so that it is still in focus)
Accommodation allows the lens power to vary in focal distance and diopter
Focal distance (f): distance from the refractive surface and the point where parallel light rays converge, in metres (m)
Diopter (D): a unit of measurement of the optical power of a lens (how strong a lens is, bend light, larger D = stronger lens = bends more light)
Lens power (P) = 1/f
ex: The distance between the cornea and focal point on the retina is 2.4 cm
Refractive power of the cornea = 1 / 0.024 m = ~42 diopters
Would a fat lens have more or less refractive power? Relate this to diopters
Fat lens = bends light more. Higher refractive power = more diopters
