Visual Optics Flashcards
What are the two ways that light can be described, and how is each used to understand light relative to the eye?
- as a wave or as a photon particle
- wave theory describes the physical changes light undergoes as it moves through the eye
- particle (photon) theory explains the energy transfer that happens when light hits the out segments of the photoreceptors
What are the two principle factors of a light wave, and define each?
- amplitude (A): maximum value of the field generated by the propagating wave (determines the wave’s intensity)
- wavelength (lambda): distance between adjacent wave crests (determines wave’e location in the electromagnetic spectrum)
What is the range of wavelength in the human visual electromagnetic spectrum?
390 nm (deep blue) to 760 nm (deep red)
True of false: the visible portion of the electromagnetic spectrum occupies a small portion of the overall spectrum?
true
True of false: many nonmammalian species, and some mammals, possess ultraviolet vision, enabling them to detect light with a wavelength shorter than 380 nm?
true
True of false, certain fish living in backwater rivers have evolved infrared vision, giving them the ability to detect light with a wavelength greater than 760 nm?
true
What other species, other than backwater river fish, has a retina that responds to infrared light?
cat (significance of this is no known)
The amount of energy in a given photon is inversely proportional to what?
its wavelength
shorter the wavelength the more intense the light, why blue light possess more energy than red light
Give a clinical example of the particle nature of light.
fluorescein staining - fluorescein sodium molecules absorb photons of blue light and reemit photos with a lower energy content in the yellow-green spectrum (process known as fluorescence)
What are the two functions of the two-part photopigment molecule found in the photoreceptor outer segments?
Part 1 - opsin: determines the wavelength of light that the photopigment will absorb (this color vision)
Part 2 - visual chormophore: uses the energy of the photon to undergo isomerization, initiating conversion of light stimulus to electrical signal.
What is the name for the process by which light energy is converted to an electrical signal?
phototransduction
What is the difference between luminous intensity and luminance?
- luminous intensity describes the intensity of a light source (measured in candela)
- luminance describes a lights brightness reflected from a surface (measured in foot-Lamberts or candela/m2)
(these two are related by not necessarily proportional - think of a street light which is intense but not very bright, and a transilluminator which is bright but not very intense)
In general, which system (scotopic of photopic) is active at luminance >3 cd/m2?
photopic
In general, which system (scotopic of photopic) is active at luminance
scotopic
What is mesopic vision?
the range of luminance during which both scotopic and photopic systems are active
How is luminance measure?
using devices called photometers
What are the two different types of photometers, and how do they work?
- visual photometers: provide a subjective reading because the observer compares the illumination of the measured light with that of a standard light
- photoelectric photometers convert the measured light into an electric current displayed by the instrument
Why are photometry measurements important in ERG recordings?
- they are used to describe such variables as threshold, ambient light, and stimulus parameters
What are the two main factors that limit the visible electromagnetic spectrum to 390-760 nm?
- absorption spectrum of the opsin component of the visual photopigment
- transmission, reflection, and attenuation of the various wavelengths by the other ocular media
Transmission, reflection, and attenuation of the various wavelengths by the other ocular media depends on what?
wavelength of light and the angle of incidence
What happens to transmission of light through the rabbit cornea from 370-500 nm to 310 nm down to 290 nm?
The amount of transmission goes from 89% to 93% down to 50%, then down to 2% at 290 nm.
Beyond what wavelength does light not transmit through the aqueous humor?
1950 nm
Explain how the aqueous and lens essentially act as filters for light passing through the cornea.
a large range in wavelengths of light pass through the cornea, and the lens and aqueous filter out the high and low ends of the range to essentially filter out UV and IR light, which protects the retina.
Does transmittance decrease through the cornea with age?
no, at least not in the cornea of the human and golden hamster
What happens to light that strikes a surface at an oblique angle?
it is reflected back, not transmitted into the new medium
What determines the critical angle for reflection?
the difference in the indices of refraction between the two mediums
Where does most of the reflection of light coming into the eye take place and why?
most takes place at the cornea because of the large difference in refraction indices between the cornea and air
What prevents the ophthalmologist form examining the iridocorneal directly in canine patients?
internal reflection of outgoing light back into the eye
How does the goniolens allow visualization of the iridiocorneal angel?
they decrease the difference in refractive indices between the cornea and air, this increasing the critical angle and permitting rays emanating from the ICA to pass through the cornea
Absorption of UV radiation has been implicated in what diseases in dogs, cattle and humans
dogs - superficial keratitis
cattle - squamous cell carcinoma
humans - cataract and macular degeneration
What happens to light entering the eye that is not transmitted or reflected?
scattered or absorbed by pigment (can have clinical applications such as diode lasers)
What three changes occur to light as it strikes denser media?
1 - velocity is reduced
2 - wavelength shortens
3 - it is bent (unless striking
What law is used to describe the amount of refraction that occurs as light passes from one medium to the other, and what determines it?
- Snell’s law
- determined by the angle of incidence and the refractive indices of both media
Define lens.
an object that bends (refracts) light
What happens when a pencil of light rays strikes a concave lens?
divergence (negative vergence) of light
The vergence power of a lens is measured in what units?
diopters
What is the equation for measuring diopters?
D (diopters) = 1/f (focal length of the lens)
What is the focal length of a lens?
Distance between the center of the lens and the point at which parallel rays of light are bent into focus by the lens
The focal length of a lens is directly proportional to what?
The curvature radius of the lens
What type of image does a convex lens create?
a real image
What type of image does a concave lens create?
a virtual or aerial image (rays are traced back behind the lens)
True of false, the vergence powers of lenses in an optical system are additive?
true
What happens to the refractive power of a lens placed in a medium other than air?
it decreases
What happens to light as it passes from the tears, through the anterior portion of the cornea, through the posterior portion of the cornea, and into the aqueous humor?
Refractive power of the tears is approximately 43 diopters, and the anterior portion of the cornea gives an additional 5 diopters, however those 5 copters are lost as light passes through the posterior cornea into the aqueous humor, which has a nearly identical refractive index to the tear film (1.336)
What is the curvature of the cornea in large eyes compared to smaller eyes?
large eyes have a flatter, less curved cornea compared to a more spherical and curved cornea
What are the two factors that affect the refractive power of the cornea?
curvature and refractive index
Ture or false, the central and peripheral cornea have the same curvatures and refractive powers?
false
After the cornea, what is the next significant refractive structure through which light passes?
pupil
When the pupil dilates, what happens to the range of distances at which objects remain in focus?
it decrease significantly
What happens to the range of focus in an eye when the pupil diameter increases from 1mm to 4mm?
range of focus goes from 0.56 - 5 meters, down to 0.78 to 1.4 meters
(this phenomenon is amplified in species such as the dog which have little accommodative capability)
As a rule of thumb, constricting the pupil by 1/2 the increases visual resolution by what factor?
2
What happens to the significance of chromatic and pupillary aberrations as the pupil dilates?
they increase
In essence, what is the pupillary light reflex balancing, and in what group of animals is this particularly important?
- maximal retinal illumination and visual resolution (increasing the one comes at the expense of the other)
- deep diving animals